The specialists say borna disease, which attacks animals' brains, could be the cause of more than half the cases of clinical depression in humans.
Hans Ludwig at the Free University of Berlin and Liv Bode at Berlin's Robert Koch Institute also say they have a drug that will wipe out the infection.
Other experts are more cautious and some have cast doubt on the findings.
In 1994 Ludwig and Bode found the borna virus in the blood of people with clinical depression. In 1996 they managed to extract it from three mentally ill patients and use it to infect rabbits, which became sluggish and withdrawn.
In January, they reported that the drug amantadine, which is used to treat parkinson's disease, stopped the borna virus from multiplying in human brain cells.
The men stress that borna disease is unlikely to be the sole cause of clinical depression, but have set up a trial in Hanover to compare amantadine with a placebo in 40 clinically depressed patients.
Juan de la Torre, a virologist at the Scripps Research Insti tute in La Jolla, California: "You have serious investigators who think the whole thing is bogus; there are others who think that the virus does cause neuropsychiatric disorders. Then there are people like me, and quite a few others, who say well, it's really too early to say anything."
Since 1985 studies have reported borna virus antibodies in anywhere from four to 40 percent of patients with mental illnesses who were under observation.
Borna, first discovered in horses, was later found to naturally infect sheep, cattle, cats and ostriches. Scientists artificially infected rats and other animals and found the virus settled in parts of the brain that regulate basic emotions.