Pathogens

A pathogen is an infectious agent or germ which is a biological agent causing disease or illness to its host. 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.
While pathogens like HIV, SARS, the Black Death, Spanish Flu and Malaria can cause millions of deaths among humans, there are others which are equally deadly for other living creatures. Most of our knowledge about pathogens which affect other species comes from the illnesses of domestic animals. DEFRA (Department for Environment, Food and Rural Affairs) has a long list of pathogenic diseases ranging from the unpleasant to the deadly. Some are potentially dangerous to humans as well and are classified as zoonoses. Eleven zoonoses are listed by DEFRA as notifiable diseases and another twenty-one as being less contagious.
It is difficult to know how serious a threat to a species' survival any pathogenic infection might be. From contemporary observation it would seem, as in the case of infected humans, some animals can recover and acquire some immunity. This would suggest that pathogens alone cannot normally be considered a cause of extinction, but with small populations this could be an important factor, particularly in association with food web disturbance or stress situations.