Studies have shown that people may be contagious for about two days before developing COVID-19 symptoms.

In fact, right before developing symptoms is when people are likely the most contagious, said Dr. Werner Bischoff, an infectious disease specialist at Wake Forest University.

People who never develop symptoms can spread infection, too. That’s a problem because many people would never seek testing unless they developed symptoms or knew they’d been exposed.

But there’s a more complicated part to this question: What if someone knows they were exposed but their virus test comes back negative -- could they still be contagious?

A negative test within less than seven days after exposure “is a very, very poor indicator of whether you have virus on board,” said Dr. Alan Wells, of the University of Pittsburgh Medical Center.

Some tests are less accurate than others, and you have to factor in the incubation period, he said.

A negative test between seven and 10 days of exposure is a better indicator, Wells said, but even then some people might not test positive until later.

If there's any question, those that have had a credible exposure, should wear a mask and self-quarantine, doctors say.

An Associated Press report contributed to this guide.