Testing only provides a snapshot of a person’s infection status, and it may not even detect cases in which patients aren’t showing symptoms. That’s why public health experts preach social distancing, mask wearing, hand washing and disinfecting as effective tools to curb the virus spread.

“There are a few problems with relying on testing as an exclusive strategy,” said Holly Fernandez-Lynch, a medical ethics professor at the Perelman School of Medicine at the University of Pennsylvania. There is a gap between testing and receiving results, leaving a window to infect others if a person does not isolate. It’s also possible for an infected person to not have enough viral material to trigger a positive result while still having the ability to spread the disease to others.

“These delays are why people are encouraged to quarantine if they think they may have been exposed, rather than waiting for confirmation of infection,” Fernandez-Lynch said. “It’s also why social distancing and mask wearing are so important.”