When a student is under the weather, it is a relatively easy decision to stay home from school; no one else is really impacted by the decision. But when a professor gets a bug, the decision to stay home or go to school is far more difficult. The professor must weigh various costs of teaching sick, including to his own health, the possibility of infecting students, staff, and colleagues, and the effect of illness on the quality of teaching, against various costs of not teaching sick, which include the loss of valuable class time and the problem of making up classes (particularly evening classes).
My own rule of thumb, put into practice just yesterday, is that as long as I feel capable of communicating at a minimal level of coherence and am not running more than a low-grade fever, I don't cancel class for illness. Frankly, I'm not at all confident I achieved a minimal level of coherence in my two classes yesterday, and the effort certainly cost me something for which I'm paying today; but I'm relieved that I don't have to figure out how to get back the class time I would have lost had I stayed home.