text: 32 questions about the coronavirus, answered — from how long patients are contagious to the potential for immunity after recovery
  - The coronavirus has sickened more than 2.5 million people and killed more than 179,000 around the world.
  - A third of the world is under some form of lockdown.
  - The US has the largest reported outbreak, representing about 32% of the world's COVID-19 cases.
  - Here's everything we know about COVID-19.
  - Read Business Insider's live coronavirus updates here, and visit our homepage for more stories.
  As the coronavirus pandemic continues to grow and evolve — more than 2.5 million people have been infected and 179,000 have died — researchers are scrambling to learn about the virus and recommend effective responses.
  Although outbreaks are still growing rapidly in many countries and researchers' understanding of the virus is changing, a consensus is evolving about some key aspects of the virus' spread, symptoms, and deadliness.
  Typical coronavirus patients develop a fever, dry cough, and shortness of breath, but these symptoms may appear at different times — or not at all.
  The virus' average incubation period is about five days, but can range anywhere from two to 14 days, according to the CDC. It's likely that people can transmit the virus during this time — research shows the average infected person spreads the virus to about 2.2 others.
  To limit that spread, many governments have declared nationwide lockdowns or otherwise dramatically restricted travel, affecting hundreds of millions of people. A third of the world is under some form of lockdown.
  Here are 32 crucial questions about the coronavirus, and what we know so far.
  Where has the coronavirus spread?
  The coronavirus has spread to nearly all of the world's countries and territories.
  At least 185 countries and territories had reported COVID-19 cases. The US, Western Europe, Turkey, Iran, and China have reported the largest outbreaks.
  What are the symptoms of COVID-19?
  COVID-19 can be similar to pneumonia — symptoms include fever and difficulty breathing.
  Gastrointestinal symptoms like nausea and diarrhea are less common, but in some cases they have appeared before any sign of fever or respiratory symptoms.
  Severe cases tend to worsen about five to 10 days after symptoms start. On average, people in Wuhan who recovered from the virus were discharged from the hospital after 2.5 weeks.
  How long does it take for symptoms to appear?
  The coronavirus' incubation period is thought to be about five days.
  The incubation period is the time that passes between when a patient gets infected and when their coronavirus test comes back positive. For the average coronavirus patient, that's five days. But in 1% of cases, it may last longer than two weeks.
  Many countries have made quarantine policies based on a 14-day incubation, but the evidence is far from conclusive. One study found that a patient's incubation period was 19 days. Another study published early in February analyzed 1,099 coronavirus cases in China and reported that the incubation could be as long as 24 days.
  How contagious is the coronavirus?
  An average coronavirus patient infects at least 2 others.
  A crucial measure called R0 (pronounced R-naught) refers to the average number of people that one sick person goes on to infect.
  The R0 for the coronavirus currently sits around 2 to 2.5, meaning infected person spreads the virus to an average of 2.2 others. So it's more contagious than the seasonal flu, but less contagious than measles.
  "It is a virus that is quite good at transmitting from one person to another," Anthony Fauci said in a February interview hosted by the Journal of the American Medical Association.
  How long is a person with COVID-19 infectious?
  COVID-19 patients can be infectious before and after showing symptoms.
  Coronavirus patients generally infect others by expelling droplets carrying the virus through speaking, coughing, or sneezing — a process known as "shedding." How contagious they are depends on how much virus they shed.
  CDC director Robert Redfield recently told NPR that symptomatic patients appear to be "shedding significant virus" up to 48 hours before their symptoms start. Research suggests that patients shed the most virus during their first week of symptoms.
  Scientists still aren't sure when a person with the virus stops being contagious to others.
  If you think you have COVID-19 and don't need emergency care, you should stay isolated until you meet three criteria: You've have had no fever for at least 72 hours (without fever-reducing medication), your other symptoms have improved, and at least seven days have passed since the onset of symptoms.
  Even when you leave isolation, you should still minimize contact with others, as well as disinfect all surfaces, clothes, and objects you've touched.
  People who develop severe cases could be contagious for longer than those with mild ones.
  Do people who have been infected develop immunity to the coronavirus? Can they get reinfected?
  Experts think people likely develop immunity after recovering from a coronavirus infection, but more research is needed.
  China, Japan, and South Korea have reported people getting reinfected. But those cases seem to be rare exceptions — and possibly the result of testing errors, experts say.
  Fauci recently told the "Daily Show" host Trevor Noah that he'd be "willing to bet anything that people who recover are really protected against reinfection."
  Since then, however, new research has raised questions about how much protection antibodies confer on people who have recovered from COVID-19. A Chinese study, which has not yet been peer-reviewed, tested 175 recovered COVID-19 patients and found that 10 of them had not developed any detectable coronavirus antibodies.
  "People don't understand the immunity to this particular virus. What we hope is if you get it once, you'll be protected against it for at least a year," Elizabeth Halloran, a biostatistician at Fred Hutchinson Cancer Research Center and the University of Washington, told Business Insider. "We don't know that, but that's what we hope."
  What does recovery from a COVID-19 infection look like?
  Recovered COVID-19 patients may have a residual cough or fatigue for weeks after major symptoms disappear.
  "It takes anything up to six weeks to recover from this disease," Dr. Mike Ryan, executive director of the World Health Organization's Health Emergencies Program, said in a March press briefing. "People who suffer very severe illness can take months to recover from the illness."
  The process is different for patients who were put on a ventilator.
  "What we're seeing in patients who end up on ventilators is that they often stay on them for several weeks," Dr. J. Randall Curtis, a professor at the University of Washington Harborview Medical Center, told US News & World Report. "And then, coming off the ventilator, they're often going to be in the ICU for several days, and then back [in a regular hospital unit] for a few days to a week or so to regain their strength."
  Though long-term effects remain unclear, patients who develop severe pneumonia could come away with scarred lungs and reduced lung function.
  When will there be a vaccine?
  Experts estimate it will take at least 18 months for a vaccine to become widely available, if not years.
  There are several dozen ongoing efforts to develop a coronavirus vaccine, according to a World Health Organization report that mentions 35 candidates.
  A range of companies, including behemoths like Johnson & Johnson and Sanofi as well as smaller biotech companies and academic research labs, are conducting research. At least seven vaccines are likely to enter the human-testing stage by the end of 2020.
  Fauci has said that the earliest the US could possibly get a vaccine would be in 12 or even 18 months. Other experts are deeply skeptical of that timeline, saying it could take years.
  Many companies are also testing existing drugs to see whether they offer promising treatments.
  What is herd immunity and are we anywhere near it?
  If enough people become immune, that can confer "herd immunity" to an entire population. The US is nowhere near that point.
  Herd immunity protects even people who aren't immune, because so many others are immune that they prevent the virus from spreading within a community. If the pathogen doesn't have the opportunity to spread, it eventually dies out.
  Populations gain herd immunity in one of two ways: with a vaccine, or through mass infection.
  To put the virus in decline, at least 50% of the population would have to be immune. Only an estimated 2-3% of Americans have recovered from COVID-19 so far. Further infection in pursuit of herd immunity would also mean far more deaths than the US has already seen.
  "You don't get herd immunity until you have a huge percent of your population that has had the disease," Melinda Gates, cochair of the Bill & Melinda Gates Foundation, told Business Insider. "We're still a long way from herd immunity. And you can't count on that, because a lot of people are going to die in the meantime if you let the experiment run and you just let the disease run its course in communities."
  Where is the epicenter of the pandemic? Which country has the largest outbreak?
  The US has the world's highest case total by far, and the state of New York is the epicenter of its outbreak.
  The US overall has reported 32% of the world's cases. The state of New York had 10% of all reported COVID-19 cases in the world as of April 21.
  As many as 100,000 to 240,000 people could die of the coronavirus in the US, even with shutdowns and stay-at-home orders, the White House's coronavirus task force warned on March 31. Without any social distancing or mitigation efforts, between 1.5 and 2.2 million Americans could die, according to modeling.
  Which other US states have the largest outbreaks?
  New York, New Jersey, and Massachusetts have the highest case counts in the US, but New York has also tested much more widely than any other state.
  All 50 US states and Washington, DC, have reported coronavirus cases. A list of each state's total is below.
  Infections have also been confirmed in the US Virgin Islands, Guam, and Puerto Rico.
  How quickly did the coronavirus spread across the globe?
  This chart shows the rate at which the coronavirus has spread worldwide since it first emerged.
  The virus was first reported in Wuhan, China, in December 2019. By February, it had appeared in other parts of Asia, Europe, and the US.
  How many people have been infected or died in China? Are those numbers reliable?
  The spread of the virus has slowed in China, but some are skeptical of its case count and reported death toll.
  China — where the virus first appeared in late 2019 — has seen a sharp drop-off in its rate of new cases, but is grappling with a wave of new infections. Authorities say that a majority of these new cases originated abroad.
  So far, China has confirmed 83,800-plus cases and over 4,600 deaths.
  However, the US intelligence community filed a report to the White House indicating that China had given "intentionally incomplete" statistics, Bloomberg News reported on March 31.
  Some Wuhan residents also dispute the official death toll, citing an increase in the shipment of urns to the city's eight funeral homes.
  "The incinerators have been working round-the-clock," one person told Radio Free Asia.
  What happened in Italy?
  COVID-19 infections overwhelmed Italy's healthcare system in March, forcing healthcare workers to make difficult choices about who to save.
  The healthcare system in Italy, which is home to one of the world's oldest populations, has been overwhelmed, with medical workers stretched thin and forced to decide where to devote limited resources.
  The flood of cases has left Italy with the world's second-highest COVID-19 death count.
  Prime Minister Giuseppe Conte placed the nation of 60 million people under quarantine on March 10. He subsequently made the lockdown stricter, forbidding all travel within the country.
  "Our response has not been perfect, maybe, but we have been acting [to] the best of our knowledge," Conte said on NBC's "Meet the Press" on April 5. "Today, I see that our model is implemented by other countries and its validity has been acknowledged by the [World Health Organization], and the results so far indicate that we are on the right path."
  How has the virus affected the rest of Europe?
  Much of Western Europe is still in the throes of the pandemic, but a host of countries are starting to lift lockdowns.
  Spain's caseload is the second-highest in the world, and the country has the third-most deaths.
  Leaders of six countries — Italy, Spain, Denmark, Austria, Poland, and the Czech Republic —imposed nationwide lockdowns in early- to mid-March, but are beginning to incrementally lift restrictions. Germany allowed some businesses to reopen on April 20, and Norway sent children back to school.
  Germany's death rate is much lower than Italy's, Spain's or France's, largely because the country has been testing an estimated 120,000 people a week.
  How many people have really had the virus?
  The true number of infected people is almost certainly far higher than the official counts.
  That's because some people can be asymptomatic and therefore may not seek a test. Some countries, like the US, have also faced test shortages.
  Some public-health experts have suggested that the actual case totals in China, Italy, and the US could be at least 10 times higher than the current figures.
  How many coronavirus tests has the US done?
  As of April 22, the US had conducted more than 4.1 million coronavirus tests.
  But the number of new tests conducted in the country each seems to be plateauing at around 150,000.
  To fully "remobilize the economy" in the US, however, a report from Harvard University experts suggests that the country should test 20 million people per day by mid-summer.
  The US Centers for Disease Control and Prevention initially designed a faulty test for COVID-19, and it then hit delays in distributing a better one for state and local labs to use, ProPublica reported. That prompted a test-kit shortage that has prevented health officials from gaining a clear understanding of exactly how many Americans have contracted the virus.
  How do various countries' testing rates compare?
  The US's testing ramp up has led to one of the highest rates of testing per capita in the world, though Italy's rate is nearly double that in the US.
  As of April 20, the US had done more than 11,800 tests per 1 million residents.
  That's a dramatic change from the situation six weeks ago. On March 8, South Korea's total number of tests done per million citizens was roughly 700 times the US's, despite the fact that the two countries announced their first coronavirus cases on the same day.
  What's the best way to prevent the spread of COVID-19?
  To prevent the spread of the coronavirus, people should wash their hands frequently, wear masks, and practice social distancing.
  The WHO, US CDC, and multiple other national health agencies all agree on the importance of handwashing. People should wash their hands frequently with soap and water, making sure to scrub for at least 20 seconds.
  People should also avoid touching their eyes, nose, or mouth with unwashed hands.
  The CDC and other authorities recommend social distancing to slow the virus's spread and prevent an influx of cases from overwhelming hospitals.
  Social distancing, according to Johns Hopkins University, consists of "deliberately increasing the physical space between people to avoid spreading illness," which includes staying at least 6 feet away from other people, canceling events, and working from home if possible.
  How many people are under lockdown worldwide?
  A third of the world's population is under some sort of travel restriction.
  India put its 1.3 billion residents under a 21-day nationwide lockdown on March 24, effectively doubling the number of people under restrictions worldwide.
  Since then, at least a third of the world's population has been under some sort of restriction. That's more people than were alive during WWII.
  "Lockdown" isn't a technical term but can include mandatory geographic quarantines, closings of certain types of businesses, and bans on events and gatherings.
  Many countries have also shut their borders, and the Trump administration has issued the highest possible travel warning for every country.
  How many schools have lockdowns closed worldwide?
  A running UNESCO tally says 191 countries had shut down their schools as of April 21.
  The closings have disrupted the education of over 1.5 billion students — more than 91% of the enrolled learners in the world.
  How long will Americans have to practice social distancing?
  Nationwide social-distancing guidelines last through at least April 30, but legal orders vary by state.
  At a White House press briefing on March 29, President Donald Trump announced that social distancing recommendations in the US will remain in effect through April 30.
  But stay-at-home and shelter-in-place orders vary across states, and some don't even have them.
  It's unclear what will happen after April. Some experts have suggested that cities and states should open things up slowly, test widely to track the virus's spread, and impose lockdowns again before new waves of infection grow too large.
  One analysis from infectious-disease researchers at Harvard, published April 14, found that social distancing measures could be necessary on and off through 2022.
  Who is most at risk of death from the coronavirus?
  Death rates for older patients are much higher.
  One study from the Chinese CDC looked at 44,000 confirmed patients in China and found that patients older than 80 had a 15% death rate there.
  In New York City, reported death rates follow a similar trend by age.
  How does the virus affect people with underlying health conditions?
  Coronavirus patients with underlying health problems are more likely to die than otherwise healthy people.
  Patients with heart disease in China had a 10% death rate, according to the Chinese CDC. The death rate for patients who reported no preexisting conditions there was less than 1%.
  Are children at high risk of getting infected or developing severe illness?
  Few children have gotten sick, but the coronavirus may pose more of a risk to kids than scientists initially thought.
  The virus mostly affects adults, but a study of 2,000 children who contracted COVID-19 in China found that children could play a role in spreading the virus and that 6% of infected kids developed severe or critical infections.
  A later study from the US CDC looked at more than 2,500 children who had COVID-19 between February 12 and April 2 and found something similar. About 5.7% of the children were hospitalized (though that was among the 20% of infected children for whom hospitalization data was available).
  The worst-affected were infants. In the Chinese study, 10% of children under 1 year old developed a severe or critical infection, compared to 7% of under-five-year-olds, and 4% of kids aged 5 to 10 years old. Of 95 infants in the CDC study, 62% were hospitalized. The estimated rate for children aged 1 to 17 was 14% at most.
  But there are likely far more children infected than we know. A recent study found that, for every one child hospitalized with a severe case COVID-19, 2,318 others could be infected. All those kids could unknowingly transmit the virus to others.
  The WHO cautions parents to safeguard their children against the coronavirus.
  How does the virus affect healthcare workers?
  Healthcare workers are particularly at risk of infection, especially when protective equipment is scarce.
  Nearly 9,300 US healthcare workers have tested positive for COVID-19 and at least 27 have died, according to preliminary data from the CDC. Those workers made up about 3% of all known cases in that time period. In some states, healthcare workers accounted for roughly 11% of the total COVID-19 cases, the CDC said.
  The report noted that was likely an underestimate, because data about whether patients were healthcare workers was only available in about 16% of cases.
  In New York City, some healthcare workers told Business Insider they aren't being tested if they come down with symptoms. Many feel poorly protected in caring for patients because of shortages of personal protective equipment.
  In China, at least 3,400 healthcare workers have been infected, and at least 46 have died. Research published in February found that nearly a third of hospitalized patients studied at the Zhongnan Hospital of Wuhan University were healthcare workers.
  Li Wenliang, a doctor in Wuhan, was hailed as a hero in China after he contracted the virus and died while treating patients. He had warned fellow medical-school alumni about the disease before it was widely understood, for which Chinese authorities initially punished him.
  When and where was the first case of COVID-19 reported?
  The first case of the coronavirus was reported in late December in the central Chinese city of Wuhan.
  However, Chinese authorities think the first case may have emerged on November 17, according to government data reviewed by the South China Morning Post. The identity of "patient zero" — the first human case of the virus — has still not been confirmed.
  How did authorities respond to the first coronavirus cases?
  In late January, officials quarantined Wuhan and nearby cities by shutting down all transportation. The restrictions are starting to lift.
  Wuhan's 11 million residents were told not to leave the city, barring special circumstances. All of Wuhan's transportation — including trains, buses, metros, ferries, planes, and cars — was halted January 23.
  A CNN analysis in February found that more than 780 million people in China — more than half the population — were under some sort of travel restriction.
  But China is now lifting restrictions.
  Some experts fear that when lockdowns end, a "boomerang effect" will occur, bringing a resurgence of the virus. Tokyo and Hong Kong, cities which managed to prevent outbreaks despite being some of the earliest-hit cities, are now worried about a new wave of cases.
  Where did the coronavirus come from?
  Some experts think the coronavirus first infected humans at a seafood market in Wuhan. But some research suggests the virus could have originated elsewhere.
  Since most of the early patients had links to one market where live animals were sold, scientists pinpointed it as the likely origin of the virus. However, a group of Chinese scientists recently published a study suggesting that the virus could have started somewhere else, with the Wuhan market merely boosting the outbreak.
  The virus almost certainly originated in bats, though an intermediary species might have enabled it to spread to humans.
  How did the coronavirus first infect humans?
  The most likely intermediary species between bats and people are pangolins, pigs, or civets.
  Researchers at the South China Agricultural University have suggested that the endangered pangolin may be the most likely candidate.
  Are there any other viruses like this one?
  SARS, which also originated in bats, is another member of the coronavirus family.
  SARS jumped to humans from civet cats at a Chinese market that sold live animals. The virus killed 774 people from November 2002 to July 2003.
  How does COVID-19 compare to SARS?
  The total number of COVID-19 cases and deaths have far surpassed those of the SARS outbreak of 2003.
  But the new coronavirus has a lower fatality rate than SARS, which killed around 10% of its 8,098 confirmed cases.
  Kieran Corcoran, Lauren Frias, Ali Millington, Rosie Perper, and Aylin Woodward contributed to this report.