There are several hundred microbes around us. Some of them are useful for human kind i.e. they add up our lifestyle in some or other way. However, many of them are not so friendly. They intended to cause harm to normal human life process to cause diseases.
Infection and immunity involve interaction between the animal body (host) and the infecting microorganisms. Based on the relationship with host, microorganisms can be classified as saprophytes and parasites. Saprophytes are free-living microbes that subsist on dead or decaying organic matter.
Saprophytes are found in soil and water and play an important role in the degradation of organic material in nature. They are generally incapable of multiplying in living tissues and therefor little relevance to cause diseases.
On the other hand parasites are microbes those establish themselves in host and multiply. Parasitic microbes may be either pathogenic or commensals.
Many commensals behave as facultative pathogens in that they can produce disease when the host resistance is lowered.
Infection may be classified in many ways, initial infection with a parasite in a host is termed primary infection. Subsequent infection by the same parasite in the same host are termed reinfections. When a new parasite sets up an infection in a host whose resistance is lowered by a preexisting infectious disease, this is termed secondary infection. When in a patient already suffering from a disease a new infection is set up from another host or another external host, it is termed cross infection. Cross-infections occurring in hospitals are called nosocomial infections.
TextBook Of Microbiology