Two German scientists have a provocative theory for how humans were first infected with HIV, prompting shouts of skepticism from scientific corners but barely a peep from the popular press. Gerhard Brandner and Werner Kloft speculate that a certain kind of biting fly could have transferred the virus from chimp to man, countering the prevailing theory of an African hunter handling raw chimp meat infected with SIV (simian immunodeficiency virus) that mutated into HIV.

The new hypothesis fingers the common stable fly, Stomoxys calcitrans, as the culprit: It spits up undigested blood from previous meals when it bites, leaving viable any viruses it might contain. One fly could have dined on SIV-tainted monkey steak and then had a second course of Homo sapiens.

Baloney! barks Beatrice Hahn, MD, the queen of HIV origins, who points to the lack of data. "I consider it naive, bordering on irresponsible," she says. "If you want to make a claim like that, you'd better come up with the data."