Definitely be extra vigilant about doing what hvert suggested. Keep in mind, too, that the chances that your apartment mate has something totally unrelated to Covid 19 are quite high. I went to get a regular flu shot about four days ago, because even though that doesn't protect against Covid 19, it protects me from flues, which are still more common right now than Covid 19. There's always severe allergy attacks, food poisoning, and many other illnesses he could have.