BONUS / A False Dichotomy: Airborne versus Droplet / Roxanne Khamsi and Lydia Bourouiba

I'm Dr. Celine Gounder, and this is Epidemic. In this episode, we're going to talk about research that raises a troubling question: How is the novel coronavirus transmitted? One of the most reassuring things we hear from experts is that it's not airborne, which we think means you can only catch it by touching something, a surface that's got the virus on it. So you wash your hands a lot, and you think you'll be okay. But last weekend, Wired magazine ran a story with the headline, "They say coronavirus isn't airborne, but it's definitely borne by air." It turns out airborne is a very technical term in this context. As the Wired story lays out, researchers who look hard at the details of how viruses travel in the air, stay in the air, and potentially cause infections through the air say there's reason for concern. I should say at the outset that this doesn't mean we should all be panicked about just walking around, especially outside. But people who spend a lot of time in indoor environments with a lot of sick people, like healthcare workers, may have a lot to be concerned about, especially because this country doesn't have a great supply of respirator masks and other personal protective equipment. In the trade, we call it PPE. So that's a worry for healthcare workers and for anybody who hopes that healthcare workers stay well themselves to care for people who get sick. I've got the author of that Wired story, Roxanne Comsey, and one of the scientists whose work she explores, Lydia Bariba, an MIT professor who directs the Fluid Dynamics of Disease Transmission Lab. I started by asking Roxanne how she became aware of Lydia's work.

I mean, I, I have to say that I have a weird fascination with sneezes. I have had allergies. I actually lived with a dog growing up. My sister's a veterinarian, and I didn't know until I went to college that I'm allergic to dogs. So I spent a lot of my childhood sneezing and having allergies and blaming it on the weather. So the fact that her research was about sneezes, it's a kind of selfish personal reason, but I'm just fascinated with sneezes. Sneezes, and the idea that you could see this through the lens of physics just wowed me. It just floored me that, that people are studying it like that. So I guess I came to it through a very selfish reason, but now I'm looking at it through a whole different lens of public health.

And then, as I understand it, you were at this meeting in 2015, and you were talking to somebody about MERS at this conference. Tell me a little bit about that conversation.

So I was at this meeting where scientists from around the world come together and just talk about viruses. And my job in the capacity as an editor and journalist is to go around and talk to people and just see what's interesting them and whatnot. And I talked to a scientist, and I remember we were standing outside in the sun, it was super hot, and I was scribbling away in my notebook because that's what I do when I talk to scientists. And he just kind of He started talking about MERS and how people don't appreciate that it is spread in air in some of the hospital systems. And, you know, I talk to a lot of scientists all the time. It's my job. There's not a day when I'm working that I don't talk to a scientist. And every once in a while, you kind of pick up what people aren't saying but are saying. And he seemed to be saying with his tone that this is something people should pay attention to that they're not paying attention to. So always in the back of my mind has been this thought with coronaviruses that, you know, that they have an underappreciated airborne nature to them that we don't fully understand, that we can't call them necessarily airborne until we know them. But as soon as this new coronavirus came onto our radar, I immediately started thinking about that conversation and I immediately started wondering, you know, how much does it behave like its cousins? Because it is a different virus, but you know, if it's the Brady Bunch, it's Marcia to Janet, you know, these folks are related but they're different. And so if one of them travels by air, it just immediately got me wondering, are we going to see reports of this being more airborne than we thought? So that's how I initially got interested. And so I guess, you know, Lydia is just really well known in her field. She's renowned for her work in studying the physics of these violent— what is it called, Lydia? You had a great title for your paper that I've forgotten.

Violent exhalations.

Yeah, exactly, violent exhalations. You know, she's world known for this. And so I remember, I think there was an article written about her, and I just thought, hey, this woman knows about sneezes and she knows about coughs and and I want to know what she knows. And I'd spoken with some of her colleagues like Linsey Marr, who's also very focused on this, and it seemed that there was a consensus amongst people who study the transmission of viruses in the air that something is here that people aren't really looking at.

And Lydia, how did you get interested in studying this of all things? I mean, it seems like a somewhat very specialized area of study.

Yeah, so the path was first in applied math and fluid dynamics, but I was in Canada, in fact, in Montreal, and SARS, as you might— may know, hit Canada pretty hard. And I was always interested in public health on the side, in addition to pursuing math and physics training and research in fluid flows. But when SARS hit, I really realized that I really strongly felt about trying to contribute in epidemiology in particular. I joined a lab in Toronto that was founded post-SARS for major coordinated efforts in modeling data integration. And what I realized relatively quickly is that the top-down modeling approaches that we were using in more classical epidemiology had its merits, but ultimately there was this major gap in scale between that kind of approach, top-down population modeling, which relies basically on epidemiology data, which often has underreporting and major gaps, and then the research being done at the molecular level, at the pathogen-cell interaction. And there is really a big gap of scale, but also of communication and ability to even bring these two sides of the research world together. And it just dawned on me that it was critical to in fact link these two sides, because we have no way right now to upscale, for example, information gained about a virus infecting one cell to what happens in the whole host when they are infected and what happens to that host in the room, and therefore how is that host affecting potentially others in that room or that building or else. And I realized that those— that scale at the intermediate junction is in fact a scale in which fluids at various forms are involved. I started seeing the connections and basically aiming to build framework that enables us to study these processes.

So Lydia, before we turn more specifically to your research, I'm just curious, you know, you were in Toronto during SARS. Could you just share in a little bit more detail, you know, how that affected you or what you observed, what you experienced during that time?

So I was in Montreal during— I mean, I was between, in transition between Montreal and Toronto during SARS. I mean, what was really clear was how unprepared we were and how quickly things could really propagate from one side of the world to the other and in unprecedented ways. And that really, that we were very, very much unprepared. And what was really striking is, particularly once we started learning more about the events of transmission, is that how little we understood about processes that involve that host-to-host transmission. But it became very clear that these are notions that we really needed to understand to inform even the healthcare workers of what they should be doing, right? There was a lot of confusion about, for example, what masks to use. There were different recommendations being given. And studies after SARS have showed that indeed there were events of transmission, for example, even with— given the recommendations that were given to the healthcare workers. And the second, I would say more than second, the other striking piece was that really the protection of the healthcare workers is critical because that's where really the frontline is. And these are basically the groups that are the most at risk and that we need to keep going. And that's what's really very striking. And so these lessons stayed with me and framed pretty much all the questions that I think about in my research related to respiratory diseases.

And by the way, Lydia, I don't know if you know, I'm in Montreal right now.

Oh, no, I didn't realize.

Ah, okay. And I moved here in December and now I can't go back to the States. Oh, okay.

Roxane, maybe if you could lay out, like, what was the question you were asking in your Wired piece?

So I wanted to know what airborne meant. What does the word airborne actually mean when scientists and virologists talk about it? Because I detected that there was a debate underlying the debate. In other words, I felt that there were people debating whether it was airborne, but part of me felt that there wasn't even consensus about what the word airborne meant, and that that could be a huge problem in both scientists talking amongst themselves, but also about the public understanding of the transmission of this disease. Because I had this underlying concern that if we don't understand what airborne is when we talk about it, we could go off the rails and see a science communication disaster, leading some people to say, for example, that public health officials are lying because it is airborne and they're saying it's not, or vice versa, a huge swath of people saying, "Chill out, it's not airborne at all." And I think that I started to detect that neither of those approaches would be truly accurate when one looks at the nuances of biology and physics and things like that. There's sometimes, you know, a— we want to put everything in boxes, right? But sometimes things don't fall neatly into boxes.

Yeah, so Lydia, that was actually my next question is, and maybe this is what you're referring to, but what is the difference between droplet, droplet nuclei, aerosol, and airborne spread of disease?

Yeah, so I think what you were referring to, Dr. Sand was the fact that these dichotomies between drops and aerosols are basically false, or this dichotomy is basically false. So that relates to your question, Céline, about airborne aerosols, droplets. So this is all basically nomenclature that was introduced to try to classify routes of transmission. And what the current definitions entail is to basically have this very separated dichotomy and categorical split between what we would consider big drops that fall faster than they evaporate and call that basically drops, but other drops that we would call then something else, in this case aerosols, would be those that in theory would evaporate so, so quickly that they would evaporate faster than they fall. And so effectively the mental image is then of isolated droplets that are coming out and so Some that fall basically very quickly and others that evaporate faster would basically stay suspended and therefore airborne. And that's something that has been debated between what are the drops that would be airborne versus those that would be large drops. And that has changed throughout history. William Wells was the scientist that really introduced this notion of large drops versus small drops through a series of experiments in the context of tuberculosis transmission in the 1930s. And at the time, Wells actually had a lot of pushback from the medical community in introducing even the concept that things could remain suspended in the air. And so Wells really was seminal in introducing, in some sense, this additional subtlety in this picture to say, well, actually, there are other things that you might not see that are also emitted and they are evaporating faster than they fall. That's why you don't see them on surfaces. And he did a series of experiments to show that. To at least support that, and eventually won. And this dichotomy between large drops and small drops, or aerosols, which is the word that is being used to distinguish them from the other drops, has been introduced. But what is being used to then classify the diseases, to say they're large drop route or aerosols, is epidemiology data, right? So we're back to trying to link host-to-host transmission to an epidemiology large population process with the issues of how that data can really be used here, right? Because we really are not really tracking necessarily all the movements and observing what these individuals have been doing when we're linking secondary cases coming from a person. So it's a probabilistic game which brings us then to the notion of distance associated with this classification. And that's another place where our arbitrary classification and use of numbers is also problematic, because we're using these 3 feet. So WHO right now is using a 3-foot safety distance to say that these droplets, or large drops if you will, do not go further than 3 feet. Other agencies or guidelines or documentations in Europe, or even the CDC, are using, for example, 6 feet. These distances are also computed from the calculation that you would emit isolated, again, drops. So if you will, you know, a ball that is being thrown out and seeing, well, how far would it go if it stays in a drop form? How far would it go? And those models have a flaw that we have highlighted in the recent work, that they're not accounting for the actual gas phase that is also being emitted during these exhalations, which is changing completely the range that these drops of all sizes can reach.

So in other words, the 6 feet distance is not enough. What are you observing once you take into account the gas phase?

Right. So in conducting the experiments where we are basically directly visualizing exhalations both from coughs and sneezes, and also, you know, heavy breathing. What is very clear is that this gas phase that's coming out is in a regime that we call in fluid dynamics turbulent regime. So it's basically high momentum. It's really a lot of energy during these exhalations. So it's not that smooth air breeze. Concretely, what we in fact not only predict with the modeling, including the cloud, but visualize directly is that the whole cloud and its payload can move coherently, so in a concentrated manner, up to distances of 7 to 8 meters, for example. So that's about 20 to 26 feet, for example, from a sneeze. For coughs, it's a little bit less. But it's certainly way more than the 3 to 6 feet.

I mean, what you're saying is really alarming to me, because that's how we have planned our infection control practices. There was a paper that was just published in the New England Journal of Medicine on aerosol and surface stability of the coronavirus, the SARS-CoV-2 coronavirus, and those researchers found that the virus could remain viable in aerosols for the duration of their experiment, which was 3 hours. So how do you integrate that with what you just told me in terms of concern for healthcare worker risk.

Right. And this is a really key point because the physics that we're deriving from visualization tell us at least how far things go and that they stay coherent. Now the other key piece, of course, is where are the pathogens in there and how long they persist. And that's why these kind of studies to also look at the duration of persistence are so critical. And so the way that you would integrate those is to really integrate the notion of distance and concentrations with time to be able to get a sense of risk of exposure in a given space. And of course, that becomes a little bit specific to which space you're in. If you are in a negative pressure room with high air changes, then the risk of exposure after X amount of time would be different than if there is basically none of that. But ultimately, at this point, the recommendation that, that would be the most conservative in terms of risk management would be to have respirators if you enter the room.

So I guess one question I have then is, you know, we're looking at hospitals being full of patients with coronavirus. If they're not already, they will be within the next week or two. Why is it at all safe for healthcare workers to be in a hospital full of coronavirus patients without personal protective equipment at all times?

If I can just jump in here, Celine, and mention some things that have been concerning to me, and I think I have tried to raise this question because it's something that I do sit with and wonder. I worry about people like you who are on the front lines, people who are medical workers, and I worry whether some of the lessons from SARS are being forgotten, and that's not an inconsequential thing. And so one thing that comes to my mind is there's a close cousin of the new coronavirus that cropped up some not so long ago called Middle East Respiratory Syndrome, or MERS. And in a study that scientists did following these outbreaks, they found that there was a 35-year-old man in South Korea during one of these outbreaks of MERS, and he was in the ER in the hospital, and about 1,600 people were in his vicinity, the scientists calculated, over the course of time that he was in this hospital, and he infected 82 people, and 33 of those people were patients, 41 of them were visitors, and 8 of them were staff. In the course of this ER visit. And, you know, obviously there was overcrowding, but it points to me that these coronaviruses, they might not just be about touching surfaces or touching your face, and that there is an aspect of them that we might be overlooking, and that that might be placing healthcare workers at risk. I mean, I'm speculating here because the data has not proven this for the new coronavirus, so I have to be careful to mention that. And I'm not a scientist, I'm not a doctor, but I am somebody who's looked at the literature and just wants to know the answers to these questions. I don't know if, you know, other people would agree, but it's, it's of concern to me. And we could also talk about what happened at some of the residential complexes with SARS and with this new coronavirus, where sewage, sewage traps or sewage plumbing that wasn't quite great seems to be the cause of the spread of these pathogens. So I think that the implications could be pretty large.

So you're referring to the Amoy Gardens apartment building, I believe. Lydia, maybe if you can explain to us what happened in that building.

I mean, as Roxanne mentioned, they suspect that the contamination was due to this failure in the trap. These traps, that's how we prevent odor. You have basically liquid that is basically covering up for the sewage that is going through the different pipes from other locations. And there was a failure that led to the aerosols that are generating from that water to be basically contaminating the other units.

So Lydia, let me ask you this. So could flush toilets, whether in the hospital or just in a public restroom, be helping to propagate this?

So at this point, I would be very careful with that because there is evidence that the virus is in the stools, but it's not clear that that is really an event that would have dominated the routes of transmission that we have seen. The physics of high-pressure flushing generates drops that are contaminating surfaces and air, and that's clear. But whether the load in those drops is significant, it's not clear. There's really not as much evidence to be able to support a bold claim about that.

So there are different medical procedures that can generate aerosols, like bronchoscopy, which is when we look down into somebody's lungs, or ventilation. What are some of the other common things, whether it's in a health care setting or elsewhere, where you might be generating aerosols like this?

Yeah, so definitely the procedures that are very much aerosol generating include also intubation. And that's gonna be the procedure that is going to be very much required for treating the worst cases, right? We really need to be thinking about what kind of PPE the staff will use and how many people can be in there and for how long. And the other side of that is, well, are there other procedures that are aerosol generating that we might want to scale down if they're not absolutely necessary.

Given what we know about the fluid dynamics and given that we know that the coronavirus can survive aerosolized for at least 3 hours, why is it okay for a healthcare worker to be working in a hospital full of coronavirus patients at all without personal protective equipment on them at all times?

And that's something that is making me very, very nervous.

The CDC recently issued guidance on its website on, quote, strategies for optimizing the supply of face masks. It has downgraded its recommendations on what masks to use from N95 respirator masks to the loose-fitting surgical masks, which are really designed to protect against splashes and sprays in the operating room and also to prevent sick people from coughing and sneezing infectious particles into the air. The CDC goes on to say that healthcare workers may reuse their face masks several times if they're in short supply and even use homemade masks like bandanas and scarves if they run out of face masks. I mean, wow, bandanas?

So the— this dichotomy between these large drops and aerosols is really at the root of what is driving these policies, right? And that's what fundamentally needs to change, because if we remain stuck in this bucket, as Roxane said, view, or basically very rigid dichotomy, then there is this belief that after 3 feet these large drops are not a risk, and therefore you can walk around with no PPE even up to 3 feet or 6 feet from the person. And so I think it is really time to realize that although convenient, because of course we want to put things in boxes and in categories, and having two of them is easy to communicate. And although, of course, relaxing that statement and saying not everything is just large droplets over 3 feet can induce some level of concern, but this is not about the general population. This is really about making the right decision at the frontline for the healthcare workers, given the room configurations and the procedures that really might even if it's not intubation, might require to really be wearing N95 respirators at all times. And from the research that I do and the literature that has been published about persistence, so these survivals, and the fact that, you know, we have direct visualizations of these clouds laden with droplets— and again, droplets here, I mean all droplets, not just the large ones that fall 3 feet, all continuous size drop distribution distribution that is trapped in the cloud and carried forward. I mean, these recommendations are not in line with the science.

So my story went online on Saturday, and then a couple days later things changed. This is evolving fast. The WHO kind of was more vocal about how it might be airborne and said that the respirators should be used for these procedures. But what I wonder about is is supply determining policy rather than science? And because there is a lack of supply or lack of availability, there's a lack of availability of proper respirators as far as I understand. And what I wonder is, okay, so if the WHO is saying you need a respirator for these procedures that we know aerosolize this stuff. What about if you're a doctor seeing somebody who's coughing a lot and you're just given a surgical mask? Is that enough? Like, I would have to obviously defer to the experts, but it's a question that I have, and I think it's a question that needs to be addressed.

So to provide a little bit more context, many of the hospitals in New York City have now run out of or have very limited supply of the N95 respirators. And so as a result, healthcare workers are seeing patients with the loose-fitting surgical masks rather than the respirator masks. And for people who are listening, just as a reminder, the respirators are the tight-fitting masks that really are meant to filter air from the outside in, whereas the surgical masks are what we put on patients so that they don't contaminate the environment around around them when they sneeze or cough. So Lydia, you know, hearing that, that healthcare providers are wearing surgical masks, not respirators, and knowing what you know about the fluid dynamics and the infectiousness of the aerosols, what would you say to that?

Yeah, so this is a problem. I mean, at this point, as you mentioned earlier, we're not at the point where, you know, the hospitals are at at overcapacity, but we will most likely get there at some point. And this is problematic. And in part, and this is not to, again, stoke fear or else, but I think that's where the decision-making and how it's done and based on what is so critical. Because as Roxane mentioned, if it's really driven by shortage and therefore we're adapting your condition based on shortage, then that's really doing things in some sense upside down, right? But because the risk clearly from what we have seen from these exhalations and from the rapidity of spread of this is really calling for respirators. And if that's what's needed, then instead of— I mean, of course, if we have no choice, then it's sort of war times. You do with what you have. But that would mean that one of the highest priorities besides diagnostics and of course developing vaccine that we would have in a year or two, is production of alternatives and high-grade respirators and diverting immense resources to make sure that we have that kind of PPE ready. And that decision would start by understanding and accepting that this dichotomy is not necessarily what should be driving these decisions right now, and that even if you wanted to stick with this dichotomy between large and small aerosols, there is just not enough data to say that this falls even in the bucket of the large drops, right, of the droplets. So that's why it's so important to have clarity in these guidelines, because they really affect that kind of decision-making. And at this point, I think that this is a mistake. We should have a recommendation of respirators to the healthcare workers particularly when we're going to have a surge of COVID patients coming in and swamping the hospitals.

In the absence of that, I'm thinking back to, you know, I was in West Africa during the Ebola outbreak, and, you know, patients were in some of those Ebola treatment units essentially being treated actually outdoors, and you would have this plastic sheeting, but it was essentially outdoor tents. I mean, is that the kind of thing we need to be entertaining in the absence of appropriate PPE for healthcare workers?

Absolutely. I mean, of course, the first choice is to get the right PPE if in any case the hospital is at full capacity, and even if it's not, I was talking this morning with, you know, a city mayor, and what I was saying is the testing being done outside, for example, even for the screening, needs to happen. It shouldn't be done in a— by some kind of triage staff inside, because it's a huge amount of individual, hopefully when we'll get enough tests that are functional, that are gonna come through. And these should be done sometimes in another location from the hospital so that we're not also exposing, because of testing, the other patients and healthcare workers in particular in the hospital setting. And in the hospital setting, if weather permits, then having these kind of natural ventilation, ad hoc solutions, either by expanding outside or retrofitting quickly certain designs of windows, like done basically in poor resource locations around the world, would be a priority.

Roxane, you've been looking into classic studies of infectious disease transmission and whether these diseases are airborne. I know you dug up a study about airborne flu, quote unquote, on a plane. Can you talk a little bit about that study?

Yeah, absolutely. Donald Milton, who is a researcher in this space as well, pointed me to this debate about whether influenza, you know, the flu, is airborne and how he still feels like he's trying to convince folks all these years later, you know, a century after the Spanish flu, that influenza can be considered, you know, truly an airborne disease or how airborne it is. And he noted that there was this kind of interesting and intriguing example of a plane that was grounded on the tarmac for 3 hours in 1979. And interestingly, the ventilation system has broken down or is not operational as they're kind of trying to address this engine failure, try to figure that out. And meanwhile, there's somebody that's on the plane sick. Lo and behold, 3 days later, within 3 days, 72% of the 52 passengers on that flight were sick. And most of those tested for the flu tested positive. So the idea is that this one person on the flight infected like 3/4 of the people on the flight in this 3-hour window, or at least in the course of the flight exacerbated by this 3-hour window in which the ventilation was kaput. And yet we're still here in 2020 with people kind of debating how airborne flu is. And I'm sure that Lidia could speak more to that debate, but it was really intriguing to me that this bizarre story from the 1970s wasn't convincing enough to most people.

Yeah, I think this is a great point. I'm glad you highlighted this because that's where you have these almost ideological blocks, right? And that has repeated itself in scientific history. Multiple times. Now we're talking, of course, about this particular concern of public health, but ultimately it becomes, you know, the dogma, right? And so, and I want to put this in perspective and return to the story of Wells, because in some sense it's the same dogma that is still sort of there. He received a lot of resistance from the medical community and was ridiculed by the medical community at the time with this idea that it was airborne in the context of tuberculosis, he then managed to really establish that very clearly. But for SARS, for example, that was also a debate. And MERS, as we mentioned earlier in this discussion, that was also a debate. And even for TB, tuberculosis, you can find still some pockets of the communities that still debate this. And I think it's something that, if anything comes out that is good out of this, I really hope that we get break through this because that's when we're gonna get out of this dichotomy that is again false dichotomy of large droplets versus aerosols that is blocking us in keeping us in some sense behind in the 1930s even though we're in 2020. So I think these debates are inherently in some sense dogmatic and the only way that you can that you can break that is to have the science and the measurements and enough of us that just continue to persist doing this kind of research so that we can finally just quantify and put the same efforts as we put in all the other areas of medicine and basically infectious disease.

So I'd be curious to hear what you guys think, each of you. Would I be wrong in saying that if I am a healthcare worker working in a hospital full of coronavirus patients, patients that I should be wearing an N95 respirator at all times?

My answer would be yes.

That that would be correct for me to do that?

Yes.

And our health systems should say to anyone who has power to move production forward or to intensify production of these respirators, like, why isn't that happening? Or why hasn't that happened? What can we do to help the situation so that there isn't shortage? Going forward.

So I have to admit, I am really anxious, not that I wasn't already, but I am really concerned having had this conversation with both of you. And just in the time that we've been talking, I've actually gotten 2 text messages from colleagues in other cities asking me, is it safe for us to be wearing a mask only some of the time? Is it safe for me to be wearing a surgical mask and not an N95 mask when I'm taking care of these patients. And I'm going to be getting back to them, and I'll be telling them they should probably be wearing an N95 mask all the time.

Yes, Celine, I'm worried. Please stay safe yourself.

Yeah, thank you.

Thank you so much for this. This is very important.

This morning, my old college buddy Rick Corcoran reached out with a question. It relates to the conversation we just had here.

Hey, Celine, it's Rick. Hey, I'm wondering about masks. Should the general public be wearing the masks that I'm seeing on the street? And if so, do I have to get one of the official N95 models, or can I just get one of the lighter-duty ones, like from Home Depot? I'm hearing conflicting reports. Thanks.

I hear you, Rick.

Eric, you're worried. You're worried about Alexa and your daughters. But here's the thing. Many hospitals have already run out of N95 masks. We're wearing surgical masks, those loose-fitting masks, and the CDC is now telling us that we may have to wear bandanas and scarves in the hospital to protect ourselves. The general public has the option to stay home. We don't have that option. We have to go into work to take care of all of you. Staying home is a much more effective way of avoiding infection than wearing a mask. So please, if you have N95 respirator masks at home, please reach out to the healthcare workers in your life and donate those masks to those who need them most. Drop them off on their doorstep outside their home. The health system is already overwhelmed. The more of us who get sick, the worse that situation's going to get. More people will die from coronavirus, and more people will die from all the other stuff they get sick with. Rick, stay safe and be well. I'm thinking about all of you. Epidemic is brought to you by Just Human Productions. Today's episode was produced by Dan Weissman and me. Our music is by the Blue Dot sessions. If you enjoy the show, please tell a friend about it today. And if you haven't already done so, leave us a review on Apple Podcasts.

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Just Human Productions is a 501(c)(3) nonprofit organization. So your donations to support our podcasts are tax deductible. Go to epidemic.fm to make a donation. Starting next week, we'll be releasing Epidemic twice a week, on Tuesdays and Fridays. But producing a podcast costs money. We've got to pay our sound engineers. We've got to pay our producers. So please make a donation to help us keep this going. And also check out our sister podcast, American Diagnosis. You can find it wherever you listen to podcasts or at americandiagnosis.fm. On American Diagnosis, we cover some of the biggest public health challenges affecting the nation today. In season 1, we covered youth and mental health. In season 2, the opioid overdose crisis. And in season 3, gun violence in America. I'm Dr. Celine Gounder. This is Epidemic.