For a long time, a simple view was taken of physical ailments. For instance, diabetes resulted from a pancreas that could not keep up with insulin production. AIDS was caused by reckless sexual behaviour. The idea was to identify exactly what was causing a problem, and fix it. If your pain is caused by a broken leg you put the leg in a cast and allow it to heal, right?
When awareness of mental illness began to grow, doctors and scientists began to notice that mental states could influence physical health. Patients who are depressed, for example, tend to recover from simple physical injuries more slowly than their peers without a mood disorder. Obviously, simple physical conditions were not so simple, after all, if they're progress could be so drastically affected by mental states.
Since mental illnesses are themselves triggered by a dizzying array of interacting causes and effects, a different way of thinking about all health problems - mental and physical - emerged.
The multifactorial model proposes that most health problems have an interconnecting set of causes rather than one distinct cause. In fact, the multifactorial model considers physical, mental, social, psychosocial, and environmental causes for health problems. For example, AIDS can indeed be caused by reckless sexual behaviour, but efforts to decrease such behaviour do not have as much of an impact as we would like, because we are not considering multifactorial causation. When you realize that a person's depression or bipolar disorder may drive them to promiscuity, or that cultural and social factors are strong influences on a person's sexual habits, you can begin to understand and tackle the real problems behind a medical issue.
The multifactorial model allowed doctors to understand the mechanisms that lead to type 2 diabetes. Rather than simply having an ineffective pancreas, sufferers of type 2 diabetes have often contributed to an effect known as insulin resistance (the insulin is in there blood stream but their body does not react to it because it has been desensitized to large amounts of insulin over the years) by the foods they have been eating, and the timing and quantity of those foods, for their whole lives. In this case, mood disorders can also be a factor, as low serotonin levels contribute to high cravings for sugar, which lead to insulin resistance and type 2 diabetes.
The multifactorial model is a more holistic approach to medical problems. It looks at a person as a whole; their history and lifestyle, rather than just one malfunctioning organ or system. This model leads to more accurate diagnosis and treatment.