In 1907, like many doctors of his day, Henry Cotton, MD, a psychiatrist, believed that infections caused by germs, a.k.a. pathogens, could generate psychological problems. Cotton used this premise to remove the decaying teeth, and even organs, of his patients to cure their mental health disorders.

Eventually, scientists exposed Cotton’s quackery, and the theory that germs caused mental illness was mostly laid to rest. But today, this scientific viewpoint is regaining credibility among some doctors, such as Turhan Canli, PhD, an associate professor of psychology and radiology at Stony Brook University. Canli believes pathogens might cause depression in much the same way a parasite called Toxoplasma gondii (T. gondii), which can live in the intestines of cats, is associated with triggering behavioral changes in those the organism infects.

Indeed, there are a number of severe mental disorders that can be triggered by the germs that cause infectious illnesses such as syphilis, rabies, mad cow disease and strep throat.

Author Harriet A. Washington explores these possible associations in her book Infectious Madness, about the numerous microbes that live in, on and around us.

These virus, fungi, bacteria and other organisms constantly adjust in type and number at different locations in and on the body, Washington explains, “and our health, including our mental health, changes with it.”