RETREATING forests may lead to more than just soil erosion, say scientists. They could also cause diseases, AIDS included.

Epidemiologists have come up with evidence linking the destruction of rainforests with the spread of deadly microbes and viruses. So far, however, these intriguing theories have remained circumstantial. A new project, led by medical anthropologist Carol Jenkins of the Papua New Guinea Institute of Medical Research, will look at how people are affected by logging and the intrusion of outsiders, reports Science.

Some virologists believe that most "new" viruses that affect humans have, in fact, been around for centuries in other host species. Rockefeller University virologist Stephen Morse explains that AIDS and yellow fever viruses were probably restricted to monkeys. The pathogens possibly lurked relatively undisturbed in their animal hosts, jumping to humans only in rare cases. However, once large numbers of humans moved into the forests, the viruses travelled fast.

One of the first cases of disease outbreak related to forest destruction emerged from Brazil's Amazon forests. When a flu-like epidemic hit the city of Belem, researchers isolated a rare virus known as Oropouche from the blood of forest workers. But it was not clear how the virus travelled from the jungle to the city. Researchers later found that midges -- gnat-like insects -- infected by the virus bred heavily on beans from cocoa plantations that had come up in the cleared forest and spread to humans.