What’s not recommended for everyone? Face masks. If you’re sick, they may help prevent you from spreading the virus, but they don’t do as much to help keep healthy people from getting sick. [Update: The C.D.C. later changed its guidance, recommending that everyone wear a face mask]

Two likeliest scenarios for an uncontained outbreak

This is not the first coronavirus to cause worldwide concern. The 2002 outbreak known as SARS (Severe Acute Respiratory Syndrome) and the 2012 outbreak of MERS (Middle East Respiratory Syndrome) both originated from this same type of virus.

This new version, “2019-novel Coronavirus” or 2019-nCoV, is at the moment thought to be more infectious than those two, but less likely to cause death. There are more than 40,000 cases (almost all in China), with more than 900 deaths linked to it. Many more cases are probably unreported, of course.

The best-case scenario for this outbreak would be containment. If China can pull this off, and other countries can keep those who are infected to a minimum, it’s possible that we could prevent widespread infection and eradicate 2019-nCoV in humans. This is what we accomplished with SARS, so it’s possible.

But as international travel becomes easier, eradication becomes harder. Should we be unable to contain the disease, and this coronavirus sticks around, it’s still not necessarily cause for panic.

It would be the fifth coronavirus that’s endemic in humans. (SARS and MERS did not become endemic.)

A recent article by Sharon Begley in STAT News laid out the two most likely scenarios for an uncontained outbreak. The first is that 2019-nCoV winds up being like the other four endemic coronaviruses, which cause less serious coldlike illnesses. Should this happen, we’ll worry for a few years as we track the rate of infections and make sure that it’s not more severe than we think. But eventually we won’t worry about it any more than we worry about which virus is causing our latest cold. More than a third of people infected with the other coronaviruses don’t even notice they are ill.

This doesn’t mean that some people don’t become sicker — with pneumonia, for example — after contracting these coronaviruses. They do. But the rates of bad outcomes aren’t usually high enough to make the news.