Depression often co–exists with other illnesses. Such illnesses may precede the depression, cause it, and/or be a consequence of it. It is likely that the mechanics behind the intersection of depression and other illnesses differ for every person and situation. Regardless, these other co–occurring illnesses need to be diagnosed and treated.

Anxiety disorders, such as post–traumatic stress disorder (PTSD), obsessive–compulsive disorder, panic disorder, social phobia and generalized anxiety disorder, often accompany depression. People experiencing PTSD are especially prone to having co-occurring depression. PTSD is a debilitating condition that can result after a person experiences a terrifying event or ordeal, such as a violent assault, a natural disaster, an accident, terrorism or military combat.

People with PTSD often re–live the traumatic event in flashbacks, memories or nightmares. Other symptoms include irritability, anger outbursts, intense guilt, and avoidance of thinking or talking about the traumatic ordeal. In a National Institute of Mental Health (NIMH)–funded study, researchers found that more than 40 percent of people with PTSD also had depression at one-month and four-month intervals after the traumatic event.

Alcohol and other substance abuse or dependence may also co–occur with depression. In fact, research has indicated that the co–existence of mood disorders and substance abuse is pervasive among the U.S. population.

Depression also often co–exists with other serious medical illnesses such as heart disease, stroke, cancer, hiv/aids, diabetes, and Parkinson's disease. Studies have shown that people who have depression in addition to another serious medical illness tend to have more severe symptoms of both depression and the medical illness, more difficulty adapting to their medical condition, and more medical costs than those who do not have co–existing depression. Research has yielded increasing evidence that treating the depression can also help improve the outcome of treating the co–occurring illness.

Source: National Institute of Mental Health