A comedo is a clogged hair follicle (pore) in the skin. Keratin (skin debris) combines with oil to block the follicle. A comedo can be open (blackhead) or closed by skin (whitehead), and occur with or without acne. The word comedo comes from the Latin "comedere", meaning "to eat up", and was historically used to describe parasitic worms; in modern medical terminology, it is used to suggest the worm-like appearance of the expressed material.

The chronic inflammatory condition that usually includes both comedones and inflamed papules and pustules (pimples) is called acne. Infection causes inflammation and the development of pus. Whether a skin condition classifies as acne depends on the amount of comedones and infection. Comedones should not be confused with sebaceous filaments.

Comedo-type ductal carcinoma in situ (DCIS) is not related to the skin conditions discussed here. DCIS is a non-invasive form of breast cancer, but comedo-type DCIS may be more aggressive and so may be more likely to become invasive.

Typical features of acne include increased secretion of oily sebum by the skin, microcomedones, comedones, papules, nodules (large papules), pustules, and often results in scarring. The appearance of acne varies with skin color. It may result in psychological and social problems.

The severity of acne vulgaris (Gr. ἀκµή, "point" + L. vulgaris, "common") can be classified as mild, moderate, or severe as this helps to determine an appropriate treatment regimen. Mild acne is classically defined by the presence of clogged skin follicles (known as comedones) limited to the face with occasional inflammatory lesions. Moderate severity acne is said to occur when a higher number of inflammatory papules and pustules occur on the face compared to mild cases of acne and are found on the trunk of the body. Severe acne is said to occur when nodules (the painful 'bumps' lying under the skin) are the characteristic facial lesions and involvement of the trunk is extensive.

Large nodules were previously referred to as cysts, and the term "nodulocystic" has been used in the medical literature to describe severe cases of inflammatory acne. True cysts are in fact rare in those with acne and the term "severe nodular acne" is now the preferred terminology.

"Acne inversa" (L. invertō, "upside down") and "acne rosacea" (rosa, "rose-colored" + -āceus, "forming") are not true forms of acne and respectively refer to the skin conditions hidradenitis suppurativa (HS) and rosacea. Although HS shares certain common features with acne vulgaris, such as a tendency to clog skin follicles with skin cell debris, the condition otherwise lacks the defining features of acne and is therefore considered a distinct skin disorder.

Oil production in the sebaceous glands increases during puberty, causing comedones and acne to be common in adolescents. Acne is also found premenstrually and in women with polycystic ovarian syndrome. Smoking may worsen acne.

Oxidation rather than poor hygiene or dirt causes blackheads to be black. Washing or scrubbing the skin too much could make it worse, by irritating the skin. Touching and picking at comedones might cause irritation and spread infection. It is not clear what effect shaving has on the development of comedones or acne.

Some, but not all, skin products might increase comedones by blocking pores, and greasy hair products (like pomades) can worsen acne. Skin products that claim to not clog pores may be labeled noncomedogenic or non-acnegenic. Make-up and skin products that are oil-free and water-based may be less likely to cause acne. It is not known whether dietary factors or sun exposure make comedones better, worse or have no effect.

A hair that does not emerge normally can also block the pore and cause a bulge or lead to infection (causing inflammation and pus).

Genes may play a role in the chances of developing acne. Comedones may be more common in some ethnic groups. People of recent African descent may experience more inflammation in comedones, more comedonal acne, and earlier onset of inflammation.

Histomoniasis (or histomonosis), also known as blackhead disease, is a commercially important disease of poultry, particularly of chickens and turkeys, due to parasitic infection of a protozoan, "Histomonas meleagridis". The protozoan is transmitted to the bird by the nematode parasite "Heterakis gallinarum". "H. meleagridis" resides within the eggs of "H. gallinarum", so birds ingest the parasites along with contaminated soil or food. Earthworms can also act as a paratenic host.

"Histomonas meleagridis" specifically infects the cecum and liver. Symptoms of the infection include depression, reduced appetite, poor growth, increased thirst, sulphur-yellow diarrhoea, listlessness, and dry, ruffled feathers. The head may become cyanotic (bluish in colour), hence the common name of the disease, blackhead disease; thus the name 'blackhead' is in all possibility a misnomer for discoloration. The disease carries a high mortality rate, and is particularly highly fatal in poultry, and less in other birds. Currently, no prescription drug is available to treat this disease.

Poultry (especially free-ranging) and wild birds commonly harbor a number of parasitic worms with only mild health problems from them. Turkeys are much more susceptible to getting blackhead than are chickens. Thus, chickens can be infected carriers for a long time because they are not removed or medicated by their owners, and they do not die or stop eating/defecating. "H. gallinarum" eggs can remain infective in soil for four years, a high risk of transmitting blackhead to turkeys remains if they graze areas with chicken feces in this time frame.

Histomoniasis is characterized by blackhead in birds. "H. meleagridis" is released in the cecum where the eggs of the nematode undergo larval development. The parasite migrates to the mucosa and submucosa where they cause extensive and severe necrosis of the tissue. Necrosis is initiated by inflammation and gradual ulceration, causing thickening of the cecal wall. The lesions are sometimes exacerbated by other pathogens such as "Escherichia coli" and coccidia. Histomonads then gain entry into small veins of the blood stream from the cecal lesions and migrate to the liver, causing focal necrosis. Turkeys are noted to be most susceptible to the symptoms in terms of mortality, sometimes approaching 100% of a flock. Diagnosis can be easily performed by necropsy of the fresh or preserved carcass. Unusual lesions have been observed in other organs of turkey such as the bursa of Fabricius, lungs, and kidneys.

Symptoms appear within 7–12 days after infection and include depression, reduced appetite, poor growth, increased thirst, sulphur-yellow diarrhoea, listlessness, drooping wings, and unkempt feathers. Young birds have a more acute disease and die within a few days after signs appear. Older birds may be sick for some time and become emaciated before death. The symptoms are highly fatal to turkeys, but effect less damage in chickens. However, outbreaks in chickens may result in high morbidity, moderate mortality, and extensive culling, leading to overall poor flock performance. Concurrence of "Salmonella typhmurium" and "E. coli" was found to cause high mortality in broiler chickens.