Each hair sits in a cavity in the skin called a follicle. Over time the follicle can shrink causing the hair to become shorter and finer. Ordinarily, the hair should grow back but in men who are balding the very small follicle ceases to grow any hair. The cause of baldness is not well understood, but is thought to be related to the genes and male sex hormones of the individual.

In the scalp, fungal infections often form circular, scaly, inflamed patches. Frequently, there can be temporary hair loss (hair returns when infection clears but if treatment is delayed and scarring results, permanent hair loss can be seen). This is a classical example of ringworm (tinea capitis) in a young child.

Pus-filled lesions (pustules) are unusual in alopecia areata, but can occur, as in this picture. The pustules are infected where medication was injected into the area.

Hair loss is referred to as alopecia. Patchy hair loss on the scalp is called alopecia areata. Complete scalp hair loss is alopecia totalis. Loss of all body hair is called alopecia universalis.

Hair loss is referred to as alopecia. Patchy hair loss on the scalp is called alopecia areata. Complete scalp hair loss is alopecia totalis. Hair loss from the entire body, including the eyebrows, eyelashes, and body hair, is alopecia universalis.

This picture shows alopecia areata, under treatment. The hair loss is being treated with anthralin, which causes the brownish hyperpigmentation and can induce hair regrowth. It is very unlikely regrowth will occur when hair is lost along the scalp margin (ophiasis).

Trichotillomania is repetitive twisting and twirling of the hair. The hair loss is usually in a well-defined area with shortened, broken-off hairs and early regrowth of hair. The scalp is the most commonly involved site, but eyelashes and eyebrows may also be involved. The hair loss can also be patchy and poorly defined.

Folliculitis, decalvans causes scarring with hair loss (alopecia). There are areas of "corn stalking" (grouped hairs arising within the area of alopecia), redness (erythema), crusting, and pustules. Due to severe scarring, permanent hair loss occurs in the involved sites.