Irritable bowel syndrome (IBS) is associated with the protozoan parasite Giardia lamblia, and pathogenic strains of the protozoan parasite Blastocystis hominis. Irritable bowel syndrome in those with HIV is associated with the protozoan Dientamoeba fragilis. IBS is also associated with the bacterium Mycobacterium avium subspecies paratuberculosis.
Parkinson's disease is associated with influenza A virus, as well as the protozoan parasite Toxoplasma gondii.