The newly identified coronavirus that emerged late last year in central China started a pandemic that’s already killed tens of thousands of people. Most deaths have been in Europe, and the U.S. now has the most confirmed cases of any country. While it took three months for the first 100,000 cases globally, the next 100,000 appeared in 12 days, and the total doubled again in barely a week. The contagiousness of the virus, which causes a lung illness dubbed Covid-19, has health experts worried it could rival the most devastating outbreaks in recent decades.

(This story updates with new data on contagiousness in section 8 and new information about the possibility of re-infection in section 9.)

1. What makes this virus so worrying?

It’s been described as “insidious” because many infected people are well enough to go about their daily business, unwittingly spreading it to others. The estimated median length of time between infection and the onset of symptoms is five to six days, and some of those who are infected don’t develop symptoms at all, including children. A study of 191 patients in China found that the median duration of infectiousness was 20 days with a maximum of 37 days. A Japanese study suggested that a substantial proportion of infections are spread by people before they fall ill. This can make it difficult to identify the people they had close contact with during that infectious period so that precautions can be taken against further spread.

2. How deadly is it?

Estimates of the fatality rate are unreliable in the early stages of an outbreak because there may be many people with mild cases who go uncounted. A number of projections have put the rate at about 1%, which would make it 10 times more deadly than the seasonal flu. A study published March 30 estimated it at 0.66%. A related coronavirus killed 9.5% of patients in the 2002-2003 epidemic of severe acute respiratory syndrome, or SARS, and another known as MERS-CoV has led to death in 34% of the 2,499 cases recorded since 2012. In those outbreaks, however, the viruses didn’t transmit from one person to another as efficiently as this new one does and didn’t spread as widely. In the worst pandemic in recent history, the 1918 influenza hit as much as a third of the world’s population, killing about 2% of those infected, or an estimated 50 million people. The last pandemic, an outbreak of a new strain of H1N1 flu (swine flu) in 2009, infected an estimated 61 million people in the U.S. alone and killed an estimated 12,469 of them in the first year it circulated.

3. What does the virus do?

It appears to cause a mild illness lasting about two weeks in children, adolescents and younger adults in most cases, and potentially more severe disease lasting three to six weeks in older people. In a Chinese study, 81% of confirmed cases were mild, 14% were severe and 4.7% critical. Frequently reported early signs are fever, dry cough and tiredness. A sudden loss of taste and smell is an early symptom among some patients. Sputum production is another sign, if the virus stays in the nose and throat, which it does for the majority of people infected. Danger starts when it reaches the lungs.

4. What are the complications?

One in seven patients develops difficulty breathing and other severe complications. Those who become critically ill typically suffer failure of the respiratory and other vital systems, and sometimes develop septic shock, according to a World Health Organization report. In an early study, more than a quarter of hospitalized patients developed a complication known as acute respiratory distress syndrome.

5. Who’s most at risk for complications?

Apparently the elderly and those with underlying health issues. Many of the fatalities have been in patients with other illnesses such as high blood pressure, diabetes or cardiovascular disease. A Chinese study of 72,000 cases found that 81% of deaths occurred in patients over 60 years old; in another study of about 4,000 cases in the U.S., 80% of those who died were 65 or older. (By contrast, in the H1N1 pandemic, about 80% of those who died were younger than 65, according to the U.S. Centers for Disease Control and Prevention.)

6. How do people contract it?

By coming into contact with virus-containing droplets that are emitted when an infected person coughs or sneezes, according to the WHO. These can be transferred directly to someone in close proximity or by touching a contaminated surface and then the mouth, nose or eyes. One study concluded that the coronavirus can survive four hours on copper, 24 hours on cardboard, 48 hours on stainless steel and 72 hours on plastic. Simple disinfectants kill it.

7. Can it spread other ways?

There’s a theoretical risk the coronavirus can spread through feces or in tiny airborne particles known as aerosols. In a study, researchers who deliberately aerosolized it under laboratory conditions found active virus can float for as long as three hours. The WHO stressed, however, that according to current evidence, the possibility of airborne transmission is limited to medical procedures with Covid-19 patients that generate aerosols, such as tracheostomies. The agency noted that in an analysis of more than 75,000 cases in China, no airborne transmission was reported.

8. How contagious is it?

Epidemiologists try to gauge contagiousness by estimating the number of additional people likely to be infected by an individual with the virus. That measurement, called a basic reproduction number or r0 (pronounced “r naught”), is one indicator of how difficult an epidemic is to control. A study of an outbreak aboard a cruise ship estimated that the r0 for the coronavirus in that case was 2.28 at the early stage. But a mathematical analysis from Los Alamos National Laboratory found the figure was 5.7 during the early epidemic in Wuhan. The Los Alamos team’s results were specific to the Wuhan outbreak. But if they hold true elsewhere in the world, the pandemic may be more difficult to control than some authorities have anticipated. By way of comparison, seasonal flu has an r0 of about 1.3 and killed an estimated 61,000 people in the U.S. in the 2017-18 season.

9. Can people catch it more than once?

That’s an important unknown. South Korea’s Centers for Disease Control and Prevention reported in early April that 51 patients had recovered and tested positive again afterwards. The head of the agency suggested that rather than being re-infected, these people had cases where the virus was reactivated. In a study published March 12, researchers theorized that it may take a few days for the immune system to completely eliminate residual viruses, which can be hard to detect. During that period, the virus may briefly rebound and, depending on the patient’s general health, there is a risk of a relapse.

10. Why is the re-infection question so important?

With many pathogens, the body develops antibodies that protect against reinfection for life. But it’s not always the case. Reinfection after a period of months or years has been found to be common with other human coronaviruses. If enough people in a community develop immunity through prior illness or vaccination, so-called herd immunity can become established. That reduces the number of susceptible people to a level at which transmission slows and eventually stops. The necessary percentage depends on the r0. For an r0 of 2.2, the threshold is about 55%; for an r0 of 5.7, it rises to 82%.

11. Could warming weather help combat the virus?

Just because outbreaks of influenza wilt with seasonal changes doesn’t mean the novel coronavirus will behave in the same way, though there is some indication it may. Some evidence suggests that as the weather warms, flu migrates to the opposite hemisphere, before returning again with cooler weather. So, even if the coronavirus does subside in the summer, it could return in the fall.

12. What is a coronavirus?

Coronaviruses are named for their crown-like shape. There’s a large family of them, responsible for diseases that range in severity from the common cold to MERS. Some transmit easily from person to person, while others do not. The WHO says that new strains emerge periodically around the globe, and several known versions are circulating in animals and haven’t infected humans.

13. Where did it come from?

The virus emerged in early December in Wuhan, an industrial city of 11 million and capital of Hubei province. Early attention focused on a seafood market where live animals were also sold, but about a third of the first 41 cases were found to have no link to it. The viral genome is closely related to several coronaviruses found in bats. Diseases transmissible from animals to humans, sometimes referred to as zoonoses, comprise a large percentage of all newly identified infectious diseases.

14. Should I be worried about my pet?

There is no evidence that domestic animals, including pets and livestock, can spread Covid-19 to people, according to officials including at the WHO and the American Veterinary Medical Association. But there have been examples of animals apparently contracting the disease from humans. One tiger tested positive at the Bronx Zoo, according to the USDA National Veterinary Services Laboratories, which said evidence suggested the animal got it from an infected zoo employee. Two house cats and two dogs living with infected owners are known to have tested positive, but Jacqui Norris, a professor of veterinary microbiology and infectious diseases at the University of Sydney, said the timing of those results also indicated human-to-animal transmission. Thousands of other animals tested have been negative. To be safe, health officials recommend that a sick person avoid contact with all other members of the household, including pets. If that’s not possible, use proper hand hygiene.

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