The three most recent outbreaks of SARS, bird flu and swine flu indicate that the next pandemic is likely to be zoonotic in origin.

ECONOMIST: Where and when will the next pandemic emerge?

Wolfe works mostly in the forests of Cameroon tracking these viruses that can jump from animals to humans - what are called zoonotic viruses.

CNN: Tracking deadly viruses' spread from animals to humans

The European Centre for Disease Prevention and Control (ECDC), which monitors disease in the European Union, said initial findings suggested the virus may have originated in animals - diseases known as zoonotic.

BBC: New 'Sars-like' not easily transmitted says WHO

The most prolific and deadly zoonotic is HIV, the virus that causes AIDS. In 1999, scientists at the University of Alabama at Birmingham traced the origins of HIV back to a subspecies of chimpanzee.

CNN: Tracking deadly viruses' spread from animals to humans