The coronavirus may take many days — up to 14 — before an infection flares into symptoms, and although most people recover without a serious illness, this is not a bug that comes and goes quickly. A serious case of covid-19 can last for weeks.

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The virus lurks in the body even after people feel better. A new study in the Lancet, based on research in China, found that the median length of time the virus remains in the respiratory tract of a patient after symptoms begin is 20 days. Among patients who survived the disease, the virus continued to be shed for between eight and 37 days. (The study did not reach any conclusions on whether and to what extent this persistence could lead to infections in other people.)

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This coronavirus can establish itself in the upper respiratory tract, said Vincent Munster, chief of the Virus Ecology Section of Rocky Mountain Laboratories, a facility in Hamilton, Mont., that is part of the National Institute of Allergy and Infectious Diseases. That enables the virus to spread more easily through coughing and sneezing and stands in contrast to another coronavirus that Munster’s laboratory has studied — MERS, which tends to infect cells in the lower respiratory tract, he said. Though more lethal than the new virus, MERS did not spread as easily and did not become a pandemic.

Munster and his colleagues have been studying the novel coronavirus under laboratory conditions to better understand its viability outside a host organism — in the air and on surfaces.

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Those experiments found that at least some coronavirus can potentially remain viable — capable of infecting a person — for up to 24 hours on cardboard and up to three days on plastic and stainless steel.

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When aerosolized into fine, floating particles, the virus remained viable for three hours. On a copper surface, it was four hours, the study found. The median length of viability for the virus on stainless steel was 13 hours, and 16 hours on polypropylene, a common type of plastic.

The new paper from Munster and his team was posted on a preprint site, and it has not yet been published in a journal.

The researchers used a nebulizer to aerosolize the virus, but in a natural environment, the virus does not spread through aerosol particles. Certain hospital treatments can result in aerosolized virus, but the main way the virus has been spreading has been through droplets — such as when someone sneezes or coughs. Such droplets can travel up to six feet.

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As the coronavirus spreads, the simple act of touching a surface has become a delicate matter of risk analysis. The world is full of suspect surfaces. Is it safe to touch an ATM screen? Or the self-checkout at the grocery store? A door handle? A package that came in the mail?

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The Centers for Disease Control and Prevention recommends that people take steps to clean and disinfect surfaces. But the durability of some coronavirus on a surface does not mean that it remains just as infectious as the hours go by. Most virus particles degrade in a matter of minutes or hours outside a living host, and the quantity of infectious particles goes down exponentially over time.

Although it is theoretically possible for a person to become infected a day or two after someone has deposited virus particles (for example, by sneezing) on a surface, it is much less likely than in the first couple of hours after the sneeze, said Munster.

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“The risk of becoming infected via these routes of transmission reduces over time,” Munster said. ‘That window of becoming infected is highest in the first 10 minutes, or one hour or two hours.”

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He addressed a commonly voiced concern: that a package in the mail may be a vector for the disease. He said that is very unlikely, but added, “There’s never zero risk if the person who gave you the package just sneezed on that package one second ago.”

During a CNN town hall program on the coronavirus Thursday night, Anthony S. Fauci, the longtime director of the NIAID, addressed the issue of packages: “I think if you start thinking about money and mail and things like that, you can almost sort of immobilize yourself, which I don’t think is a good idea.”

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A virus is a peculiar object that is inert and arguably not truly alive outside a host. Only when it invades a cell and hijacks its machinery can the virus begin to replicate.

Outside, on an inanimate surface, the virus will gradually lose the ability to be an infectious agent. It may dry out, for example. It can degrade when exposed to ultraviolet radiation from the sun. A person sneezing on a surface may deposit many thousands of virus particles, and some may remain viable for days. Still, the likelihood of a person who comes into contact with the remnants of that sneeze goes down over time, because most infections are the result of a large viral load.

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“The more exposed you get to more virus, the higher the likelihood it is that you get infected,” Munster said.

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That was echoed by Gary Whittaker, an infectious disease expert at Cornell University, who said that typically it takes “an army of viruses going in” to break through the natural defenses of a human being, which include mucus that lines airways.

“We’re talking about thousands or tens of thousands of particles to infect an animal or a person,” Whittaker said.

The U.S. strategy for slowing the spread of the coronavirus is currently focused on social distancing, in recognition of the fact that human beings are vectors of covid-19. Simultaneously, people in the United States have been urged by the CDC to clean and disinfect surfaces in their homes. The CDC has put forth guidance on how to blend a disinfectant solution from bleach — five tablespoons (1/3 cup) of bleach per gallon of water (and never mix bleach with ammonia or any other cleanser).

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Amid these precautions, people should understand that surfaces that contain the virus — known to scientists as fomites — are not the major drivers of this pandemic. Covid-19 is primarily spread through direct person-to-person contact.