It assumes genes have a major part in mental health. You have a pre-dispositioned to a certain disorder. Kendler found relatives of schizophrenics were 18x more likely to get the disorder
Chemical imbalances in the brain may be involved in certain mental illness'. The neuro transmitter, Dopamine has been found to be excessive in schizophrenic patients. There is a cause and effect problem with this factor though.
It is common in physical illness but also found in some mental illness. Barr et al found that increased levels amongst schizophrenics of their mothers having flu whilst pregnant is a possible link. Syphilis (an S.T.I) caused brain damage
Abnormality can occur if the brain is damaged in some way. Alzheimers is caused by loss of cells and damage to the nervous system. Alcohol and drugs can cause hallucinations which are symptoms of a mental health disorder. Korsakoffs syndrome; impairment of memory caused by alcohol abuse