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A human pathogen is a pathogen that causes disease in humans.
The human physiological defense against common pathogens is mainly the responsibility of the immune system with help by some of the body's normal flora and fauna. However, if the immune system or "good" microbiota are damaged in any way , or antibiotics being taken to kill other pathogens], pathogenic bacteria that were being held at bay can proliferate and cause harm to the host. Such cases are called opportunistic infections.
Some pathogens have been responsible for massive numbers of casualties and have had numerous effects on affected groups. Of particular note in modern times is HIV, which is known to have infected several million humans globally, along with the influenza virus. Today, while many medical advances have been made to safeguard against infection by pathogens, through the use of vaccination, antibiotics, and fungicide, pathogens continue to threaten human life. Social advances such as food safety, hygiene, and water treatment have reduced the threat from some pathogens.