Alopecia universalis

Alopecia universalis is the most severe form of alopecia areata, an inflammatory disease of the hair follicle, which is characterized by a complete loss of hair of the scalp and all the hair-bearing areas of the body.
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Alopecia totalis

Alopecia totalis is a form of alopecia areata, an inflammatory disease of the hair follicle, characterized by a complete loss of hair of the entire scalp which becomes glabrous.
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Alopecia areata

Alopecia areata is an autoimmune disease that affects almost 2% of the US population, including more than five million people in the United States alone. In alopecia areata, the affected hair follicles are mistakenly attacked by a person's own immune system (white blood cells), resulting in the arrest of the hair growth stage. Alopecia areata usually starts with one or more small, round, smooth bald patches on the scalp and can progress to total scalp hair loss (alopecia totalis) or complete body hair loss (alopecia universalis). Any hair-bearing site on the body can be affected by alopecia areata. Hair loss can often occur without any accompanying symptoms.
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Alopecia areata

Source: National Institute of Arthritis and Musculoskeletal and Skin Diseases

What Causes It?

In alopecia areata, immune system cells called white blood cells attack the rapidly growing cells in the hair follicles. The affected hair follicles become small and drastically slow down hair production. Fortunately, the stem cells that continuously supply the follicle with new cells do not seem to be targeted. So the follicle always has the potential to regrow hair.
Scientists do not know exactly why the hair follicles undergo these changes, but they suspect that a combination of genes may predispose some people to the disease. In those who are genetically predisposed, some type of trigger—perhaps a virus or something in the person’s environment—brings on the attack against the hair follicles.