Depression may not be genetic or a result of poor nutrition, obesity or other causes, a new study indicates. Rather, major depressive disorder may be caused by parasites and bacteria, and one psychology professor wants the disorder re-classified as an infectious disease and suggests research for a vaccination.

With an estimated 15 to 20 percent of the population that claims to be depressed at one time or another, a prominent psychology professor says major depressive disorder may be attributable to parasitic invasion and could possibly be cured by a vaccination.

Several examples are given by Turhan Canli, PhD, associate professor of Psychology and Radiology at Stony Brook University that back up his claim that depression may be caused by bacterial, parasitic or viral causes.

In his study, called "Biology of Mood and Anxiety Disorders," Canli said that despite decades of research efforts, major depressive disorder still remains among the most prevalent of mental disorders. Canli says that pharmacological approaches have not changed and have brought only temporary relief to patients. The recurrence rate of depression is 50 to 80 percent in patients, indicating illness goes untreated, he wrote.

The study indicates that research shows markers present in patients, and in the postmortem brains of mood-disordered patients, are consistent with those that ward off pathogens produced by microorganisms. 

Parasites, bacteria and viruses have been proven to impact brain function, as several previous studies point out.  

A parasite that lives in cat excrement has been proven to change the brain functions of rats and mice so they lose their fear of the smell of cat urine. A study conducted by UCLA showed that women who consumed probiotics found in yogurt showed altered brain function while in a state of rest, Brain Balancers reported.

A virus, called ATCV-1, which was found in algae, has proven to slow brain functions, specifically in people who were taking tests that required visual processing, according to Science Mag.

"It would be worthwhile to conduct large-scale studies of carefully characterized depressed patients and healthy controls, using gold-standard clinical and infectious disease-related study protocols, as have already been developed for bacteria and viruses," Canli wrote. "Such efforts, if successful, would represent the 'end of the beginning,' as any such discovery would represent the first step toward developing a vaccination for major depression."