Morgellons (/mɔː(ɹ)ˈdʒɛlənz/) is the informal name of a self-diagnosed skin condition in which individuals have sores that they believe contain some kind of fibers. Morgellons is poorly understood but the general medical consensus is that it is a form of delusional parasitosis; the sores are the result of compulsive scratching, and the fibers, when analysed, turn out to originate from textiles.

The name was coined in 2002 by Mary Leitao, a mother who rejected the medical diagnosis of her son's delusional parasitosis. She revived it from a letter written by a physician in the mid-17th century. Leitao and others involved in her Morgellons Research Foundation successfully lobbied members of the U.S. Congress and the U.S. Centers for Disease Control and Prevention (CDC) to investigate the condition in 2006. CDC researchers issued the results of their multi-year study in January 2012, indicating that there were no disease organisms present in people with Morgellons and that the fibers found were likely cotton, and concluded that the condition was "similar to more commonly recognized conditions such as delusional infestation".

Morgellons is poorly understood but the general medical consensus is that it is a form of delusional parasitosis in which individuals have some form of actual skin condition that they believe contains some kind of fibers.

Secondary organic delusional parasitosis occurs when the state of the patient is caused by a medical illness or substance (medical or recreational) use. In the DSM-IV this corresponds with "psychotic disorder due to general medical condition". Physical illnesses that can underlie secondary organic delusional parasitosis include: hypothyroidism, cancer, cerebrovascular disease, tuberculosis, neurological disorders, vitamin B12 deficiency, and diabetes mellitus. Any illness or medication for which formication is a symptom or side effect can become a trigger or underlying cause of delusional parasitosis.

Other physiological factors which can cause formication and thus can sometimes lead to this condition include: menopause (i.e. hormone withdrawal); allergies, and drug abuse, including but not limited to cocaine and methamphetamine (as in amphetamine psychosis). It appears that many of these physiological factors, as well as environmental factors such as airborne irritants, are capable of inducing a "crawling" sensation in otherwise healthy individuals; however, some people become fixated on the sensation and its possible meaning, and this fixation may then develop into delusional parasitosis.

Secondary functional delusional parasitosis occurs when the delusions are associated with a psychiatric condition such as schizophrenia or clinical depression.