When it comes to the new coronavirus, just who is a ‘close contact’?

Even as U.S. authorities have taken the drastic steps of quarantining residents returning from China, and temporarily banning foreign visitors who recently traveled to affected Chinese regions, they have urged the vast majority of U.S. residents to go about their regular activities.

But there are exceptions. People who returned from China on or after Feb. 3 have been formally quarantined or asked to stay home. And behind the scenes, local public health officials have launched painstaking efforts to reach “close contacts” of people with confirmed cases of the virus, dubbed 2019-nCoV, asking them to self-quarantine and submit to ongoing monitoring.

So what exactly is a “close contact”? It’s an intuitive-sounding term, but has a clinical definition that varies by infectious disease. The deadly measles virus, for example, can linger in a place for hours, in the air and on surfaces, after an infected person has spent time there. Coronaviruses are generally passed through droplets, requiring close physical contact with a sneeze or other body secretion.

The European Union’s health agency offers a descriptive definition: someone living in the same household as a patient; someone who has had face-to-face contact with or been in a closed environment with a patient; or a health care worker directly caring for a patient. It’s also any plane (and by extension, train or bus) passenger sitting within two seats, in any direction, of an infected person. The World Health Organization flags health care providers and household members, as well as anyone who has been within 3 feet of a confirmed case once the infected person had symptoms.

As for the rest of us, even casual contact with an infected person, such as crossing paths on the street or briefly being in the same room, is thought not to pose much of a risk, though CDC officials stress there is still a lot to learn about the new virus. And the best protection in those instances also can ward off other unwanted visitors this time of year: flu and colds. The latter are often the result of four other coronaviruses that are responsible for a good chunk of winter illnesses.