Depression is a primary characteristic of several mental illnesses, which could be considered disabilities, according to the National Institute of Mental Health. Disabling mental illnesses related to depression include major depressive disorder, bipolar disorder, psychotic depression, seasonal affective disorder, postpartum depression and persistent depressive disorder.
Common symptoms of depression include sadness, anxiety, feeling guilt or remorse, loss of interest in pleasurable activities, overeating or not eating enough and thoughts of suicide, explains the National Institute of Mental Health. Fatigue, restlessness, insomnia and difficulty concentrating are also associated with depression. These symptoms can seriously impair an individual's ability to function in society and are very disabling in severe cases.