From CNN Medical Correspondent Dr. Steve Salvatore

NEW YORK (CNN) -- Schizophrenia is a rare and often misunderstood disease. It is not what many think it is -- split or multiple personalities.

The brain disorder affects about 1 percent of the world's population, including 4 million Americans.

Doctors don't know exactly what causes schizophrenia, but they believe it is a combination of biological, genetic, chemical and environmental factors.

Schizophrenia strikes both men and women equally, usually starting in the late teens and early 20s. Symptoms can start and disappear quickly or gradually.

The symptoms vary greatly from person to person and the disease is often linked to other problems, such as substance abuse or paranoia.

Many patients have trouble distinguishing between what is real and what they perceive as reality.

Mental health experts divide the symptoms into several catagories, including what doctors call "positive symptoms." They include distorted perceptions, beliefs or behaviors such as hallucinations of the senses. Doctors say these symptoms usually respond well to treatment.

So-called "negative symptoms" are marked by greatly reduced speech, energy and body language. Patients may have unresponsive facial features and make poor eye contact. These symptoms are more difficult, but not impossible, to treat.

The final category includes "disorganized" symptoms -- confused and disorganized speech, erratic behavior and a short attention span. Doctors say these symptoms of schizophrenia are toughest to treat.

The disorder can prove dangerous for some -- especially when symptoms of paranoia combine with the delusional symptoms of schizophrenia. In fact, doctors say paranoid schizophrenics are notorious for discontinuing the treatments which help control their symptoms.

New medications are often effective in helping to treat schizophrenia, but complete treatment requires psychotherapy along with social and vocational rehabilitation.

And because schizophrenia is a life-long illness, success demands the support of families and communities.