Having a transmissible illness is a good excuse to stay home from school or work: it means that whatever bug you have spreads very easily to other people.
The dangerous thing about viruses like polio, smallpox, Covid-19, and the flu is that they're extremely transmissible, spreading easily from person to person. Depending on the pathogen, this happens when a sick person sneezes, coughs, or touches a surface with germy fingers. The Latin root of transmissible is transmittere, "transfer or send across," from trans-, "across," and mittere, "to send or throw."