Scientists know that people are contagious in the early stages of infection with COVID-19, but just how contagious and for how long remains unclear.

Although a lot of emphasis has been placed on the need to stay home if you feel sick, many studies have shown that people are often contagious before they know they have been infected, making the virus that causes the disease much harder to contain.

In one recent study, researchers from China tracked 94 COVID-19 patients and found that they shed the most virus — and were likely the most contagious — just before or as they developed symptoms.

Infected people with no symptoms are likely to be just as contagious as those with symptoms, such as a dry cough, fever, and aches, according to very early research, says William Hanage, PhD, an epidemiologist at the Harvard T.H. Chan School of Public Health.

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