A pathogen, germ, or bug was a disease-causing biological agent.
According to Doctor Phlox, germs were "among the most resilient organisms known to medical science." They could survive almost anywhere -- on kitchen counters, under fingernails, and in some cases the vacuum of space.
By 2151, over 200 million space-dwelling microbes had been cataloged. One such microbe called Polycocyx astris lived inside grains of interstellar dust and was one of the most virulent species, which could drift in a dormant state for millions of years and still cause a cold. (ENT: "Breaking the Ice")
Some germs spread with personal contact. In 2373, Elias Giger refused to shake hands with Jake Sisko, as it might spread germs. (DS9: "In the Cards")
Icheb was genetically altered by the Brunali to carry a pathogen that was lethal to the Borg. (VOY: "Child's Play")