Spores allow a time outside, awaiting the next host. Coughing and diarrhea are conditions induced by the parasite that encourage its dispersal. Some pathogens, such as the spirochetes that cause syphilis, are transmitted sexually, never being required to leave a human body. It may surprise you that a number of the key organelles that make up our cells originated as symbiotic bacteria that formed long-lasting partnerships with our cellular ancestors. The endosymbiotic theory of cellular evolution has become a foundation principle of modern biology.
Bacteria outweigh all other organisms lumped together.