What are the causes and risk factors for mental illness?

One frequently asked question about mental illness is if it is hereditary. Most mental disorders are not directly passed from one generation to another genetically, and there is no single cause for mental illness. Rather, it is the result of a complex group of genetic, psychological, and environmental factors. Genetically, it seems that more often than not, there seems to be a genetic predisposition to developing a mental illness. Everything from mood, behavioral, developmental, and thought disorders are thought to have a genetic risk for developing the condition.
Medical conditions may predispose an individual to developing a mental illness. For example, depression is more likely to occur with certain medical illnesses. These "co-occurring" conditions include heart disease, stroke, diabetes, cancer, hormonal disorders (especially perimenopause or hypothyroidism, known as "low thyroid"), Parkinson's disease, and Alzheimer's disease. While it does not appear that allergies cause depression or visa versa, people who suffer from nonfood allergies have been found to be somewhat more vulnerable to also having depression compared to people who do not have allergies. Some medications used for long periods, such as prednisone, certain blood pressure medicines, sleeping pills, antibiotics, and even birth control pills, in some cases, can cause depression or make an existing depression worse. Some antiseizure medications, like lamotrigine (Lamictal), topiramate (Topamax), and gabapentin (Neurontin), may be associated with a higher risk of suicide. Despite the impact that taking certain medications or having a medical illness can have on a person's emotional state, clinical depression should not be considered a normal or natural reaction to either issue. It should therefore always be aggressively treated.
Environmentally, the risks of developing mental illness can even occur before birth. For example, the risk of schizophrenia is increased in individuals whose mother had one of certain infections during pregnancy. Difficult life circumstances during childhood, like the early loss of a parent, poverty, bullying, witnessing parental violence; being the victim of emotional, sexual, or physical abuse or of physical or emotional neglect; and insecure attachment have all been associated with the development of schizophrenia as well. Even factors like how well represented an ethnic group is in a neighborhood can be a risk or protective factor for developing a mental illness. For example, some research indicates that ethnic minorities may be more at risk for developing mental disorders if there are fewer members of the ethnic group to which the individual belongs in their neighborhood.
Stress has been found to be a significant contributor to the development of most mental illnesses, including bipolar disorder. For example, gay, lesbian, and bisexual people are thought to experience increased emotional struggles associated with the multiple social stressors associated with coping with reactions to their homosexuality or bisexuality in society. Unemployment significantly increases the odds ratio of an individual developing a psychiatric disorder. It almost quadruples the odds of developing drug dependence and triples the odds of having a phobia or a psychotic illness like schizophrenia. Being unemployed more than doubles the chances of experiencing depression, generalized anxiety disorder (GAD), and obsessive-compulsive disorder.

http://www.medicinenet.com/mental_health_psychology/page3.htm