The reasons are many and varied. The first one TightSlot has already mentioned - sitting near someone who may have a cold or who may be just starting with one and who show no outward signs but are infectious. Then we have the mucky so-and-sos who do not wash their hands properly (if at all) after using the toilet so daub their infected handprints all over the place and as germs are invisible to the naked eye nobody can see them to avoid coming into contact with them themselves.