From Reporter Louise Schiavone

(CNN) -- New research from Germany indicates some cases of serious depression may be caused by a virus.

"We think that there is ... a lot of evidence that Borna virus has clinical significance for this type of disease," said Dr. Liv Bode of the Robert Koch Institute in Berlin.

In the United States, at least 17 million people have some form of clinical depression -- not just a passing case of the blues, but a disabling and often long-term disease.

Scientists are still unraveling the causes of the disease: genetics, stress and possibly a virus.

The virus was first identified in the late 1800s among horses near the town of Borna, Germany. The horses stopped eating, walked in circles and got sick. Some even killed themselves.

Autopsies led scientists to the virus in the region of the horses' brains that controls emotions. Researchers in Berlin have found a similar strain in humans.

"I think it is supporting our hypothesis that this virus, this particular agent, has really something to do with this type of disorder," Bode said.

The anti-viral drug amantadine, used to treat Parkinson's disease, has been found to relieve some cases of depression. A trial is now under way.

"I think I'm one of the most skeptical people around ... but I have to face the fact that for about a year now, we've been treating patients and we've been seeing responses to amantadine," said Dr. Ron Ferszt of the Free University of Berlin.

German patient Rosemarie Wenzlaff, who suffered from depression for 10 years, says the medication changed her life.

"I didn't take care of myself when I was depressed; I couldn't eat," she said. "Now I'm thinking of cooking marmalade. I listen to music now. I watch TV. It's a totally different life."

Scientists in the United States say these early findings are interesting, but not conclusive. Results are expected later this year from clinical trials in Berlin that might demonstrate a link between the Borna virus and depression.