The general evolutionary tendency is for bugs to get milder in the symptoms they produce. A bug which kills everyone in short order, or which makes them feel so ill that they stay indoors and go to bed, and thus isolate themselves, will not be very good at transmitting itself from host to host. The ideal. from the bug's' point of view. is to produce coughing and sneezing which transmits the bug, but not to make the victim feel so ill that they isolate themselves.