But it’s probably not realistic to ask every one of the President’s potential contacts to quarantine and get tested. So how do health workers determine who’s at serious risk?

First, they have to assess the time period when the president might have been infectious. With reports that he was showing symptoms as early as Wednesday, the most dangerous time probably started two to three days before that, experts say.

“If you’re being really conservative, you might go five days back,” says Justin Lessler an epidemiologist at Johns Hopkins Bloomberg School of Public Health. “After the first couple of days, the further you go back, the less likely it is that he was, you know, actually infectious to anybody.”

And what really counts as a contact? Does sitting outdoors at a Rose Garden event put you at risk of catching a virus from the President at the podium? The CDC definition of a contact is someone who was within 6 feet of the infected person for at least 15 minutes. Those contacts are the highest risk, Lessler says.