In the second pathway, we see an initial spillover from animals,
followed by human to human transmission.
Ebola falls into this category, along with bubonic plague,
Dengue fever, and some strains of influenza, as well as yellow fever.
The final zoonotic pathway occurs when human exposure to an animal
pathogen results in the evolution of a specialized human pathogen that's
only transmitted from human to human, after that first transmission event.
For this category, opportunities for cross-species transmission may be common,
but successful establishment of the pathogen may be rare.
Examples include AIDS, measles and smallpox.
Our ability to respond to emerging zoonoses is further complicated
by the fact that these pathogens are emerging at an ever-accelerating rate,
due to increased overlap between humans and wildlife.
This increased overlap can be facilitated by human encroachment into wilderness
areas, that can be associated with commercial ventures such as logging,
mining, oil exploration, and industrial agriculture.
Or with the movement of wild animals into areas where people are already living.
This often happens as natural habitat is lost and animals expand their range in
search of food and mates, often into areas that are dominated by human activity.