Emerging Infectious Diseases
Today’s bio-diversity crisis is not just one of lost habitats and extinct species. It is also a crisis of emerging infectious diseases (EID’s), such as HIV in humans, Ebola in humans and gorillas, West Nile virus and Avian Influenza in humans and birds, chytrid fungi in amphibians, and distemper in sea lions. There is every reason to take these events seriously, because EID’s appear to have a long evolutionary history.
This is because many pathogens are capable of infecting a range of hosts, but evolved in places where only some of those hosts live. Pathogens also have specialized means of transmission from host to host. If, for example, a pathogen is transmitted by an insect that lives in the tops of trees, susceptible hosts will not be infected if they never leave the ground.
For humans, HIV, Ebola, West Nile virus, and Avian flu are only the latest in a long line of EID’s. When our ancestors moved out of the African forest onto the savannah more than a million years ago, they rapidly became effective predators. Sharing prey with pre-existing carnivores, they acquired tapeworms that originally inhabited only hyenas, large cats, and African hunting dogs.