A new study has said that West and Central Africa are emerging as the major sources for the next infectious disease to plague man.
Deforestation in these regions is forcing wild animals that are a natural host for pathogens into ever smaller areas and into ever likelier contact with fast-growing human populations, it said.
The paper, published in the British journal Proceedings of the Royal Society B, looks at how new, killer diseases such as AIDS, Ebola and bird flu have leapt the species barrier to humans in the past three decades.
Its authors found that closely related primates -- monkeys, chimpanzees, gorillas and humans -- pose the biggest risk of "host shift" as they share similar biology and immune responses, and are vulnerable to many of the same microbes.
The similarity is especially strong with chimpanzees, our closest genetic relatives, with whom we shared a common ancestor about 8.6 million years ago.
Humans are almost four times likelier to share pathogens with chimpanzees than with colobus monkeys, which branched from the family tree 34.4 million years ago, says the study.
The virus for acquired immune deficiency syndrome was probably transmitted to humans from a chimpanzee infected with a simian form of AIDS, previous studies have said. More than 25 million people have died of the disease since it was first reported in 1981.
But being distantly related is not a safeguard, either, the new study says.
Some pathogens, especially viruses, are smart at adapting to a new host in close proximity.
"Bird flu, West Nile virus and Hendra virus are all viral diseases that have jumped large evolutionary distances to infect humans," said lead researcher Jonathan Davies of the University of California at Santa Barbara.
Avian influenza and West Nile virus have a natural reservoir in birds, while bats provide the host for Hendra virus.
"We suggest hotspots of future emerging diseases may be found where humans come into close proximity with wild primates, as is increasingly the case in the forests of Central and West Africa, due to rapidly growing human populations and scarcity of resources," said co-author Amy Pederson of the University of Sheffield, northern England.
"In addition, we are likely to see an increase in outbreaks of novel viral diseases as humans invade previously isolated habitats, and these may be just as likely to jump from a rat or a bat, as an ape."
Predicting host shifts could slash the risk, they said. Money could be invested in projects to prevent human-animal contact and in building early-warning networks to detect any disease outbreak.
In February, a paper published in the British journal Nature found that the emergence of new diseases had roughly quadrupled over the past 50 years.
Its authors named the biggest potential source for a new animal-borne disease, or zoonose, were East Asia, the Indian sub-continent, the Niger delta and Africa's Great Lakes region.
The 2002-3 outbreak of severe acute respirary syndrome (SARS), which originated in Chinese bats, cost 30 billion dollars although the death toll was fewer than 800, according to World Health Organisation (WHO) figures.