But the president’s illness still raises more questions than it answers. Although Hicks spent considerable time with the president this week, the timing of her illness and of the president’s positive test result may not match up. COVID-19 has a regular course of disease, with a predictable number of days separating infection, early symptoms, and the worst illness. Even the best PCR tests, which detect the virus’s genetic material, are not likely to catch infections that began within the previous two days. In other words, it typically takes four or more days for the virus to multiply and reach detectable levels inside the body.

Someone can test positive for the virus without experiencing symptoms. The Centers for Disease Control and Prevention says that symptoms of COVID-19 are most likely to begin four to five days after exposure, but they have been observed to start anywhere from two to 14 days after exposure. At the same time, current evidence suggests that people who have COVID-19 are most infectious at the very moment their symptoms begin.

It’s possible, in other words, that Hicks infected the president and the first lady on Tuesday, and he tested positive yesterday. But other possibilities seem just as likely: Perhaps Hicks was sick for longer than she knew, and she infected the president earlier this week. (If so, was she infectious when she attended the presidential debate on Tuesday?) Or perhaps some other person infected the president, the first lady, and Hicks—a scenario that would indicate a very serious outbreak in the White House. It would not be surprising if, in coming days, we learn that more members of Trump’s retinue and Cabinet have contracted the virus. Nor would it be shocking to learn that the president was contagious when he shared a debate stage with Biden.