<P> There have been some inconsistent findings of links between infection by the parasite Toxoplasma gondii and human mental disorders such as schizophrenia, with the direction of causality unclear . A number of diseases of the white matter can cause symptoms of mental disorder . </P> <P> Poorer general health has been found among individuals with severe mental illnesses, thought to be due to direct and indirect factors including diet, bacterial infections, substance use, exercise levels, effects of medications, socioeconomic disadvantages, lowered help - seeking or treatment adherence, or poorer healthcare provision . Some chronic general medical conditions have been linked to some aspects of mental disorder, such as AIDS - related psychosis . </P> <P> The current research on Lyme's disease caused by a deer tick, and related toxins, is expanding the link between bacterial infections and mental illness . </P> <P> Research shows that infections and exposure to toxins such as HIV and streptococcus cause dementia and OCD respectively . The infections or toxins trigger a change in the brain chemistry, which can develop into a mental disorder . </P>

Biological factors appear not to be the most important causes of which level of mental retardation