CDC guidelines say if you've been in close contact with someone who's tested positive for COVID-19, you should get tested and self-quarantine at home.
But many people who get a negative test result rely on that and incorrectly think it means they're in the clear.
A local epidemiologist clears up why those actions could lead to more spread of the disease.
The White House outbreak continues to broaden. President Trump's press secretary Kayleigh McEnany - who previously reported testing negative - announced via Twitter she has now tested positive for the coronavirus.
Infectious disease specialist Dr. Michael Ben-Aderet with Cedars-Sinai says transmission occurs when people who know they've been exposed don't isolate because they have a negative test result.
"A full infection-control strategy that relies only on testing is going to fail here and there," he said. "When you get exposed to someone that's positive for coronavirus that has disease you don't immediately turn positive. It takes time for the virus to infect you."
Just like other respiratory diseases, incubation can take anywhere from two to 14 days. During that time you are shedding virus so you can infect others.
"It's not totally predictable, what one person's exposure period or one person's incubation period will be. It's also not predictable if someone will develop symptoms or remain asymptomatic, and it's not totally predictable if someone will have a severe disease," Ben-Aderet said.
A day before she tested positive, McEnany was still on the job talking to reporters without a mask.
"Universal masking is just a no-brainer. We've seen in multiple studies that when everyone's wearing a mask transmission rates go way down."
The CDC just added new guidance citing evidence that the virus can spread beyond six feet indoors. Ben-Aderet's advice is to stay apart, wear a mask and don't just rely on testing.
"We need to be following the recommendations and if we do, we can expect to really cut down on the transmission of this disease. And if we don't, we're going to continue to see spreading events like this."