Deciding that you are too sick to work is hard. You could tough it out, but you don’t want to expose your co-workers. The problem is that most of us are wrong about when to stay home, said Thomas Fekete, chief of infectious diseases at Temple. “The most contagious period is at the beginning, before you get really sick.” So, if you go to bed feeling slightly sick, and wake up feeling a bit under the weather, that’s the day to stay home. The following days, when you feel sicker, are days when you are less likely to share your contagious illness.