As COVID cases creep up in the Bay Area, some precautions are staring to be reintroduced. The BART board, for instance, voted on Thursday to reinstate a mask mandate on trains and platforms until the middle of the summer. Some forecasts are indeed worrisome: A model developed at the Mayo Clinic projects that U.S. COVID cases will double in the next two weeks — albeit from a low base, and still at levels well below the December-January omicron peak. For those who do catch the virus, what happens in the body even if symptoms never appear? We talked to scientists about how asymptomatic COVID runs its course.