 Among the almost infinite varieties of microorganisms, relatively few cause disease in otherwise healthy individuals.  Microorganisms that cause disease are known as pathogens.  Infectious disease results from the interplay between those few pathogens and the defenses of the hosts they infect.Extracellular pathogens take the things they need to survive and reproduce from the nutrients present in tissue fluids, or they feed directly on the cells. Infectious Diseases  Pathogens within cells are protected from many of the host’s defense mechanisms, like antibodies.