Strict (professional) pathogens are microorganisms specifically adapted to a pathogenic lifestyle. These pathogens are only capable of surviving in their host, except for short periods outside (such as on a fomite or other transmission method). The inability to survive outside of their host for long periods of time is due to the loss of genes necessary to produce molecules that can be obtained from the host. Treponema pallidum (syphilis) is an example of a strict pathogen. This STI is capable of infecting an individual’s genito-urinary tract, before moving on to immune-privileged tissues. The slow growth of this bacteria allows it to avoid causing an immune response, allowing syphilis infections to progress over decades, with a 30-40% mortality rate. Syphilis can only be transmitted by contact with contaminated blood or organs, and through unprotected sex.
Opportunistic pathogens are present in many, if not most, individuals of a species, and do not cause disease in most cases. However, when the microbiota of an individual is disturbed (such as with antibiotics or vaginal douching), the opportunistic pathogen is able to colonise the area and deploy its virulence factors. This allows an infection to be caused by an otherwise harmless pathogen. For example, Staphylococcus aureus is a bacteria found on human skin. Under normal conditions, it acts as a commensal with no harmful effects. However, if the skin is sterilised, it may be possible for S. aureus to become pathogenic and cause a skin infection. This is also able to occur where skin is not sterilised prior to puncture (such as a cut or injection), potentially leading to blood infections (sepsis). As S. aureus is so common, it is easy to acquire it into the skin microbiota through contact with a contaminated fomite.