Before December 2019, SARS-CoV-2 — the virus that causes the Covid-19 illness — was unknown to science. “A pneumonia of unknown cause” was first reported to the World Health Organization on December 31, after a slew of cases appeared in Wuhan, China. Since then, the virus has been detected in millions of people worldwide, and killed hundreds of thousands.

This is a pandemic, a global crisis and tragedy on a scale that’s hard to fathom.

There are a lot of new things for the public to learn — about the virus, controlling its spread, social distancing, treating the sick, and how our governments should react to this chaotic situation. It’s like we’ve all been dropped in to study for a test in a class that no one signed up for. It’s confusing and hard to process.

Related 11 questions about the coronavirus economic crisis you were too embarrassed to ask

Here, we’ve tried to distill it into 12 key aspects of this crisis that everyone should understand. That said, we don’t cover everything, and there’s still a lot of uncertainty about some of the most important things on the list. (If you think you have Covid-19, and are curious about what that means for you, read this explainer. If you’re most confused about the financial crisis the virus is causing, read this one.) This guide focuses mostly on the properties of the virus, and the implications for public health.

In the few months since this virus emerged, we’ve learned an immense amount about it. Scientists have decoded its genetics and whom it’s more likely to kill, and have started working on vaccines that could immunize humans to it. Still, its newness means that a lot of the figures presented in this article are estimates and subject to change as scientists learn more. So keep that in mind, too.

1) SARS-CoV-2 is very contagious, and a huge portion of the global population is vulnerable

One thing we do know is that this coronavirus, SARS-CoV-2, is very contagious. Just look at the headlines: Covid-19, the disease caused by the virus, has now infected nearly a million people around the globe.

Scientists quantify the contagiousness of a disease with a figure called R0 (pronounced R-nought.) “The figure refers to how many other people one sick person is likely to infect on average in a group that’s susceptible to the disease (meaning they don’t already have immunity from a vaccine or from fighting off the disease before),” Vox’s Julia Belluz explains. An R0 of 2, for example, means each infected person is expected to spread the virus to two others, on average. Covid-19 is currently believed to have an R0 between 2 and 2.5.

That makes it more contagious than the seasonal flu. With the flu, there are people in the population who have some level of immunity to it — either because of a vaccine or because they have been exposed to that strain of flu in the past. That’s not the case here.

With no mitigation, a statistical modeling report from the Imperial College of London found that 81 percent of the populations of both Great Britain and the US could be infected over the course of the pandemic.

The R0 is hard to estimate, even in a pandemic, because “the R0 is not a property of the virus,” explains Dominique Heinke, an epidemiologist in Massachusetts. The virus’s genetic code, the proteins that surround it, and the symptoms it causes are properties of the virus. The R0 is “a combination of the properties of the virus, and the way that humans interact.”

Also important to know: R0 is the rate of transmission within a fully susceptible population, when no control measures are put into place. “One misconception about the R0 is that it is a fixed number — but actually, it varies over time and over context,” says Caitlin Rivers, a professor at the Johns Hopkins Center for Health Security. “So it has been well established that the R0 with this disease, at least before we implement any interventions, is well over 2.”

But our actions can bring this figure down. This is sometimes called the Re, or effective reproduction rate. “If we can get Re [to be less than] 1 through social distancing, natural immunity, or a vaccine, then we break the cycle of transmission and the epidemic will slow or be suppressed,” Heinke says.

2) The virus is believed to be spread mainly by respiratory droplets

Why has the virus spread so fast? The WHO recently put out a statement saying its experts believe Covid-19 is primarily transmitted by respiratory droplets. When infected people breathe, cough, or sneeze, they expel little droplets of moisture that contain the virus. Another person in their vicinity could breathe in these particles and get infected. The virus-laden droplets can also land on surfaces that others may touch (and then get infected by touching their mouth, nose, or eyes). Scientists now believe the virus can remain viable on a hard, non-porous surface like plastic or steel for around three days, and a rough surface like cardboard for about a day.

The 6-feet-away social distancing guideline is meant to keep people out of the splash zone for these respiratory droplets. (But know there’s no hard cutoff for how far the viral droplets can spread. A sneeze can propel material from the nose 20 feet or more, a recent MIT study found.)

It’s also unknown how significant other modes of transmission are in spreading the disease. There are two other possible routes being explored: fecal-oral and airborne.

Fecal-oral transmission occurs when a virus spreads, well, through feces (usually by contaminating water or food through improper hygiene). Scientists have noted the presence of the virus in some feces of infected people. The Centers for Disease Control and Prevention says, though, that “the risk [of fecal-oral transmission] is expected to be low based on data from previous outbreaks of related coronaviruses.” (That said, if you weren’t already: Please wash your hands enthusiastically after defecating.)

You may have heard that the new coronavirus isn’t “airborne” — meaning that unlike extremely contagious diseases like measles, it’s unlikely to linger in the air for hours on end. But that doesn’t mean the virus can’t linger in the air for some amount of time.

As Wired explains, although some experts say the novel coronavirus isn’t airborne, that’s based on a narrow scientific definition of the term. The virus can possibly still linger in the air for some time and under some conditions. We don’t yet know precisely what those conditions are. It will definitely be in the air in the moments after an infected person sneezes or coughs, but it’s unclear when the particles eventually come to rest on the ground (or on surrounding surfaces).

Overall, it doesn’t appear that airborne contagion is a big factor with this outbreak. “In an analysis of 75,465 COVID-19 cases in China, airborne transmission was not reported,” the WHO reports. But it warns that “airborne transmission may be possible in specific circumstances and settings.” Certain medical procedures like intubation, suctioning, and ventilation could possibly create airborne viral aerosols. Because these occur in hospital settings, it’s extra important for health care workers to wear proper personal protective equipment (PPE) to make sure they don’t get sick.

Another big reason Covid-19 has spread so far and wide is that people with mild symptoms — and even people without symptoms — can spread it. (Fever, cough, and shortness of breath are recognized by the CDC as common symptoms, but others such as loss of smell and taste have been added to the list, and symptoms may take up to 14 days to appear.)

There are no firm estimates on this, but it seems that somewhere between 25 and 50 percent of people infected with the virus show no symptoms. Some percentage of those asymptomatic cases can spread the virus to others. (We’ll need more widespread surveillance testing to better understand the exact rate of asymptomatic cases.)

On March 13, the journal Science published an analysis that concluded 86 percent of all the Covid-19 cases in China before January 23 were not detected by public health authorities at the time (though that doesn’t mean these cases were asymptomatic). It’s estimated that these undiagnosed cases infected 79 percent of the total cases. The results suggest that, at least, the bulk of transmission of the virus in China was spread by people not sick enough to get the attention of doctors.

The fact that this virus can spread sneakily and silently makes it extra dangerous. It’s also possible to spread it before a person develops symptoms.

On April 1, the CDC published a report on how presymptomatic spread created clusters of many new cases in Singapore. In one case, the virus appeared to have spread via a church pew. Two people who had recently visited China attended church services on January 19, when they had no symptoms (they would develop them in the following days). Three other people who attended the church that day got sick, the CDC found — one of whom sat in the same seat as the people who visited China. Another instance of presymptomatic spread was found in a singing class. In others, presymptomatic cases spread in households.

3) Covid-19 is deadly

The exact death rate of the virus is another figure that’s still being calculated — and is changing all the time. Here’s the latest: A new paper in Lancet Infectious Diseases found the case fatality ratio (the percent of confirmed infections that die) to be 1.38 percent, drawing from Chinese data (which may or may not be complete). The authors also report an estimated infection fatality rate — this is the percentage of people who become infected, including those who have mild or no symptoms, who eventually die. This figure is lower, at 0.66 percent.

But there’s a huge amount of variability on this figure country by country, and demographic by demographic. South Korea, on the one hand, is estimated to have a death rate of less than 1 percent. Italy’s seems to be, for now, several percentage points higher.

The Lancet Infectious Diseases paper found that globally, the case fatality rate for those under age 60 was 1.4 percent. For those over age 60, the fatality rate jumps to 4.5 percent. The older the population, the higher the fatality rate grows. For those 80 and over, Covid-19 appears to have a 13.4 percent fatality rate.

These likely will not be the final estimates of the fatality rate for Covid-19. Like R0, “the mortality rate is not a characteristic of the virus,” Heinke explains. “The mortality rate is a function of both the severity of disease that the virus can cause and treatment. ... The mortality rate may increase as hospitals become more and more overwhelmed with cases.”

And the figure changes over time. In late February, the WHO estimated the rate in Wuhan was 5.8 percent. Now, the estimated death rate for Wuhan — the city where the outbreak began — is 1.4 percent, per a March 9 study in Nature Medicine.

“It feels like you should be able to just divide deaths by cases, and whatever that number is feels like it should be CFR,” or case fatality rate, Rivers says. But it’s not that simple. “If there are a lot of people either in the numerator or denominator that aren’t being recognized, it can change your answer a lot.” With testing lagging in the United States, it’s hard to know how many cases there are overall, and therefore, it’s hard to estimate the overall fatality rate.

One thing we do know: Older people, and people with underlying conditions, are much more likely to die of Covid-19 than younger people.

4) The pandemic in the US will get worse before it gets better

On March 31, the White House’s coronavirus task force presented grim statistics: Under the best-case scenario for mitigation of the Covid-19 pandemic, there may be between 100,000 and 200,000 deaths in the United States.

Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, emphasized then that the outbreak and its toll was going to get worse before they get better. The US has since seen daily confirmed cases and deaths fall, but experts say reopening plans may be premature.

A recent study in The Lancet found that, on average, the duration from symptoms to death is 17.8 days (and that recovery can take nearly 25 days). Other studies have shown it can take a week for the disease to progress from the start of symptoms to a person needing hospitalization. Some people just getting infected now in places without strict social distancing measures could die a month from now.

Look at the Wuhan line on this new graph from @jburnmurdoch. The lockdown was introduced there on 23rd Jan – 69 days ago – which means this entire Wuhan curve has happened since then. It shows how long it can take to see the effect of control measures on the number of deaths. pic.twitter.com/MAsBIzFevj — Adam Kucharski (@AdamJKucharski) April 1, 2020

Epidemiological modeling is useful in helping the public think through what’s possible with the disease. It also can help hospitals understand how much staffing and equipment they’ll need (and where they are likely to fall short).

Independent experts say the models the task force is using are sound: 100,000 to 200,000 deaths “certainly seem to be within the reasonable framework,” said Bill Hanage, associate professor of epidemiology at Harvard T.H. Chan School of Public Health, in a call with reporters Tuesday morning. “I would not be particularly surprised by them, I wouldn’t be surprised if they were higher, I wouldn’t be actually surprised if they were lower. One of the things that’s difficult for those of us who’ve been involved with modeling this is communicating the amounts of uncertainty that we have.”

The CDC recommends several measures to help prevent the spread of Covid-19: Wash your hands often for at least 20 seconds.

Cover your cough or sneeze with a tissue, then throw it in the trash.

Clean and disinfect frequently touched objects.

Stay home as much as possible, and do not go out if you are sick.

Wear at least a cloth mask in certain public settings.

Contact a health worker if you have symptoms. Guidance may change. Stay informed, and stay safe, with Vox’s coronavirus coverage hub.

Modeling is not an ironclad prediction of the future, however. “Unlike the weather, which we’re all accustomed to and incorporating forecasts into decision making, with pandemics we actually influence the outcome,” Rivers says. Collective actions — continuing to social distance, self-isolating if you’re sick, supporting health care workers around the country, raising the capacity of the health care system — over the next few weeks will affect whether the models are updated for better or for worse.

“I think it’s key not to get fixated on the exact numbers,” Heinke adds. “You can look at a range of models and say, we can expect it to be at least this bad.”

5) There’s a huge range of severity of illness

Young people may not be dying of Covid-19 in high numbers, but they are still at risk for severe, debilitating disease.

As of March 16, the CDC reports that 12 percent of known Covid-19 cases in the US have resulted in hospitalizations, and 20 percent of those hospitalizations were among those ages 20 to 44. But this was in the early stages of the US outbreak, so this data may be refined in the coming weeks.

In New York City, which as of April 2 has the biggest Covid-19 outbreak in the United States, 9 percent of more than 7,700 hospitalizations were for those ages 18 to 44 as of March 31.

The bottom line: In all age groups, Covid-19 can present with a huge range of severity. Many people — an exact figure is still not known — can get the virus and show no symptoms. Others become near deathly ill.

Scientists have some clues as to what puts someone more at risk than another.

“Patients with underlying health conditions and risk factors, including, but not limited to, diabetes mellitus, hypertension, COPD, coronary artery disease, cerebrovascular disease, chronic renal disease, and smoking, might be at higher risk for severe disease or death from Covid-19,” the CDC found in a recent report.

But there are still a lot of unknowns about what else contributes to risk. Even among older people, there are unanswered questions: Why do men appear to be dying at higher rates than women? Differences in severity of illness could also relate to the amount of the virus a person has been exposed to. Health care workers, when they are infected, seem to get sicker than their age cohort should suggest, which could be because they are exposed to higher doses of the virus.

Scientists will soon start to search for genetic clues, too, that might signal a person is more at risk than another. The reason might have to do more with their own biology than the virus: In some people, an excessive immune reaction called a “cytokine storm” is what leads to the most severe symptoms and death, not the virus itself.

“It’s really an open question to try to figure out why some of these younger people are getting really, really severe disease, and if there are other risk factors that we are not appreciating,” Angela Rasmussen, a Columbia University virologist, said in March. “Some of that will just have to wait until we have really detailed clinical data on all the cases that are coming out now in Italy and in the United States.” Knowing who is most at risk, she says, “will help in terms of flattening the curve.” If we learn how to protect the young people most at risk and keep them out of hospitals, we can decrease the strain on health care systems.

6) The US is still not doing enough testing

The US finally made real progress on testing in May, but experts say more is needed to fully control the outbreak. It’s worth remembering why hundreds of millions in the country are under stay-at-home orders, with infection rates soaring across the country.

“The facts remain that we wasted a lot of time in terms of ramping up testing,” Saad Omer, director of the Yale Institute for Global Health, says. The US could have spent a lot more time diagnosing and isolating the sick, and tracing their contacts, which could have prevented at least some of the spread.

Testing in an outbreak provides two functions. One is to diagnose those who are sick. The other is surveillance: to see where the virus may be lurking, especially in cases where symptoms are mild or don’t manifest at all. Many doctors have told patients with milder symptoms to just stay home and not get tested.

The US has caught up after some of the early stumbles. The Atlantic, which has been aggressively tracking the testing situation across the country, reports that around 104,000 people are being tested a day. But “testing backlogs have ballooned, slowing efficient patient care and delivering a heavily lagged view of the outbreak to decision makers,” the Atlantic’s Robinson Meyer and Alexis Madrigal write.

America needs strict social distancing measures in place, in part, because it’s unknown exactly where the virus is. And it’s unknown because testing still lags.

People need to stay in under the assumption that anyone they come into contact with could be infected. “The classic epidemiological approach to controlling disease is not to shut down society; it’s to target the people you know to have the disease and understand who they’re spreading it to,” Jeremy Konyndyk, a senior policy fellow at the Center for Global Development, says. “We can’t do that right now because we don’t have enough testing to know who has the disease.”

Not only does the US need more testing, it also needs testing that can be completed within minutes. These rapid tests are being ro: Recently, the Food and Drug Administration approved one that can give results in five minutes. It’s now being rolled out in parts of the West Coast.

The US has rolled out other kinds of testing, too, like serology — testing of people’s blood for antibodies to the virus. That way, health professionals can figure out who has already had the disease and is now immune and can safely return to be in contact with others. (Though scientists still need to do more work in determining what immunity looks like in any given person.)

7) Lockdowns and shelter-in-place orders are not an overreaction

Most Americans were under some sort of lockdown or shelter-in-place order from their local authorities. For many of them, the best thing they can do right now is just be patient — especially if their states are reopening before meeting guidelines for doing so safely.

There may be a lag of a few weeks before changes in state measures are reflected in new case and death data.

Many people are still getting sick and requiring hospitalization, who were infected before these orders went into effect. The virus can incubate for up to 14 days before symptoms appear, and then it can be several more days before severe illness sets in.

All of this is reason to keep social distancing: The outright lockdowns of movement in some cities, as well as the less severe policies in place across the country, can still dramatically slow the spread of Covid-19. And per at least one poll, people are, by and large, complying.

Also know: This may get worse again before it gets better.

8) The situation for hospitals and health care workers is dire

First, the US had a shortage of diagnostic tests for Covid-19. Now there’s also a shortage of personal protective equipment and supplies — from medical masks and gowns to hospital beds and ventilators. And the fear is there will soon be a shortage of health care workers responding to the disease. In Italy, more than 60 doctors have died of the disease.

As Vox’s Dylan Scott, Umair Irfan, and Jen Kirby report, “Doctors and nurses are reporting gear shortages, lax protocols, and a high level of stress in their workplaces — with the worst still to come. Some of them told Vox that they consider getting infected with the coronavirus an inevitability.”

Health care workers may even be putting themselves at higher risk of severe disease just by doing their jobs. “It’s not just that [health care workers] are getting infected at higher rates; instead, they’re getting sicker than one might expect on the basis of their age,” says Peter Hotez, the dean of the National School of Tropical Medicine at Baylor College. It’s possible they’re being exposed to higher concentrations of the virus, which may lead a person to more severe illness.

Meanwhile, the protective equipment that might stop them from getting sick is in short supply. Particularly acute is the supply of N95 respirators, which stop health care workers from inhaling viral-laden droplets and aerosols.

9) America can end social distancing, but an aggressive new plan needs to be in place

It’s understandable that some — maybe most — people want life to go back to normal already.

But “If we all just went right back to how things were before, transmission would start again with the same intensity,” Rivers says. To ease off social distancing, the US needs an aggressive new plan in place to prevent the explosive growth in new cases it’s currently seeing.

“Social distancing is basically a sledgehammer,” Konyndyk, who has worked on past outbreaks, like Ebola, says. “You’re just stopping everything and hoping that in the process you will also slow transmission.” What the US needs to do, he says, is turn that sledgehammer of social distancing into a scalpel: widespread testing and contact tracing.

Once there’s widespread testing, there needs to be a huge team of public health workers in place to trace the contacts of those who test positive. Everyone who tests positive or who has come into contact with someone who tests positive then needs to be put into quarantine or isolation, to not spread the virus further. This is how authorities routinely beat outbreaks — even of incredibly infectious diseases like measles.

Rivers, along with former FDA Commissioner Scott Gottlieb and other co-authors, released a plan Sunday for how to ease off social distancing. The plan has several phases: Phase one is slowing the spread through social distancing while ramping up testing capacity and ensuring hospitals have the equipment they need. In phase two, social distancing restrictions ease while public health workers continue to track and isolate cases.

But it will take a lot to get there. The authors argue these decisions need to be made on a region-by-region basis: Phase two should only begin after 14 days of sustained case reductions in an area, and only after testing capacity is dramatically increased. Even in phase two, they stress, if cases go up again, the US will need to go back to severe social distancing. Before there’s a vaccine, everyone will have to remain vigilant.

10) The pandemic is painful for millions — not just those who are sick with Covid-19

Outbreaks don’t just affect those who get sick with the illness and die — there’s also a lot of collateral damage. Unemployment is skyrocketing. Health care bills are piling up. Businesses are furloughing or laying off workers.

As the outbreak progresses, it will expose the cracks in American society and the country’s preparedness for future outbreaks. Americans need to remember the lessons learned over the next several months.

Also important: the collateral medical damage. Hospitals are postponing elective surgeries (ones that are planned in advance) and some other treatments. There could be limited resources to treat others if another disaster strikes, like a flood or a hurricane.

11) Some parts of life may return to normal. But this won’t be over until there’s a vaccine and widely available treatments.

The ultimate goal in stopping a pandemic is a safe and effective vaccine that can prevent people from getting the virus. The good news is that these are already being tested. The bad news is that it could take a year or more to find one that is safe and effective. “Honestly, I think the vaccine in 12 to 18 months is a moonshot,” Tara Smith, a Kent State University epidemiologist, says.

In the meantime, a treatment might be discovered sooner. The World Health Organization is facilitating a multinational clinical trial, testing medicines — and combinations of medicines — to treat Covid-19. But even those drugs wouldn’t necessarily stop the outbreak.

“It would be really great, I think, for saving lives,” Rivers says. “But you wouldn’t really expect it to slow transmission at all.” People could still be getting sick and spreading the virus. And everyone would need to be vigilant, and patient, in this scenario, too. Even if the risk of severe disease and death is reduced, if the number of cases increases, more people can still get sick and die.

12) The coronavirus came from nature, jumping from animals to humans. Researchers fear this will happen again.

The novel coronavirus outbreak presents an immense immediate challenge for global health. But it’s also part of a larger pattern: Viruses that circulate in animals keep jumping over to infect humans. The story of the novel coronavirus is the story of HIV, of SARS, of Ebola, and even measles. These are all diseases that have been introduced to humans — with deadly effects — via animals. And as humans encroach more and more into animal habitats, it’s believed these spillover events may only grow more common.

Scientists know this virus jumped from an animal to a human, but they are not sure exactly how or where. “If you don’t understand where it came from, then it’s hard to make policies, procedures, to prevent it from happening again,” says Krutika Kuppalli, an infectious disease physician and Emerging Leader in Biosecurity fellow at the Johns Hopkins University Center for Health Security.

It likely started with a bat — the genetics of the novel coronavirus suggest that it did. As Vox’s Eliza Barclay reported:

What researchers have to figure out now is how exactly the coronavirus jumped to humans: perhaps through a human eating an infected animal, or through humans being exposed to infected feces or urine. “All we know [is] its likely distant source was bats, but we don’t know who was between bats and people,” said Vincent Racaniello, a professor of microbiology and immunology at Columbia and host of the This Week in Virology podcast. “It could be a direct infection [between bats and humans] as well.”

A lot of the evidence points toward the outbreak either starting or significantly gaining steam at a live animal market in Wuhan, China. The more we know about how this virus jumped from animals to humans, the more authorities can help make sure an outbreak with this origin doesn’t spiral out of control and spread around the world again.

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