The interaction of humans with disease-causing bacteria is often thought of in terms of a host-invader interaction. However, there are many types of human-microbe interactions, so we need a more complex understanding of micro-organisms and their roles in normal human health and disease processes. It is useful to think in evolutionary terms. The various types of human-microbe interactions are the result of hundreds of millions of years of interactions between animals and micro-organisms. There are 3 main types of pathogen: bacteria, viruses and fungi. Not all bacteria and fungi are pathogens - pathogens are microbes that cause disease.
Strict pathogens
Some (relatively few) microbes can infect essentially all human hosts who are exposed to the particular microbe AND cause essentially the same sort of infection and disease symptoms in every infected person. “Strict pathogens”
examples: Treponema pallidum (syphilis); HIV (AIDS); Plasmodium vivax (malaria).
Opportunistic pathogens
Most microbes are more efficient at infecting some people than others, and many microbes can cause several different types of disease depending on the type of infection (for example, depending on the site of infection) and variations in host-microbe interactions. “Opportunistic pathogens”. These kind of microbes became pathogens in some conditions, for exemple, the cas of immunodeficiency.
example: Escherichia coli, Pseudomonas aeruginosa.
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