Although the exact cause of feline acne is unknown, some causes include:

- Hyperactive sebaceous glands

- Poor hygiene

- Stress

- Developing secondary to fungal, viral, and bacterial infections

- Reaction to medication

- Drinking from plastic containers to which the cat is allergic

- Demodicosis or mange, causing itchiness and hair loss

- Suppressed immune system

- Hair follicles that don't function properly

- Rubbing the chin (to display affection or mark territory) on non-sanitized household items

- Hormonal imbalance

- Contracting the infection from other cats in the same household

A study in Sweden showed that 21.5% of smokers and 3% of nonsmokers (genetic pigmentation or unknown cause) had lesions that could be classified as an oral melanin pigmentation. A gingival melanin index in 4 degrees was established. Already with a consumption of 1-3 cigarettes a day 9.3% of all 20.333 examined showed a smoker's melanosis. Pipe smokers had smoker's melanosis in 16.8%. One year after the start of cigarette smoking a clinically visible smoker's melanosis could be seen in 12.3% of women, and 17% among men.

In cigarette smokers who quit smoking, the number of individuals with smoker's melanosis becomes slowly less frequent after 2–3 months, but can still be seen in a few former smokers three years after smoking stop.

Although clinically visible genetic melanin pigmentations in the mouth are present in several ethnic groups all over the world, more mucosal areas will be melanin-pigmentet if tobacco products are used. Smoker's melanosis is found in India, Italy, Japan, Nigeria, Sweden, Turkey, USA, and several other countries.

Smoker's melanosis is expected to be found also in other tissue surfaces exposed to tobacco and tobacco smoke, for instance lips and in skin of the fingers holding the cigarette. Future studies will also show if the use of tobacco exaggerates the pigmentation of skin.

HPV is spread by direct and indirect contact from an infected host. Avoiding direct contact with infected surfaces such as communal changing rooms and shower floors and benches, avoiding sharing of shoes and socks and avoiding contact with warts on other parts of the body and on the bodies of others may help reduce the spread of infection. Infection is less common among adults than children.

As all warts are contagious, precautions should be taken to avoid spreading them. Recommendations include:

- cover them with an adhesive bandage while swimming

- wear flip-flops when using communal showers

- should not share towels.

Plantar warts are not prevented by inoculation with HPV vaccines because the warts are caused by different strains of HPV. Gardasil protects against strains 6, 11, 16, and 18, and Cervarix protects against 16 and 18, whereas plantar warts are caused by strains 1, 2, 4, and 63.

Smoking or the use of nicotine-containing drugs is the cause to Smoker's melanosis. Also tar-components (benzopyrenes) are known to stimulate melanocytes to melanin production, and other unknown toxic agents in tobacco may also be the cause. These chemical agents have a polycyclic, chain-like structure. Environmental tobacco smoke from parents is causing smoker's melanosis in their children Swedish snuff causes a small elevation of oral melanin pigmented individuals from 3.0% to 4.7%. Nicotine tablets have shown to stimulate to melanin pigmentation of the oral mucosa.

An escharotic is a substance that causes tissue to die and slough off. Examples include acids, alkalis, carbon dioxide, metallic salts and sanguinarine, as well as certain medicines like imiquimod. Escharotics known as black salves, containing ingredients such as zinc chloride and sanguinarine containing bloodroot extracts, were traditionally used in herbal medicine as topical treatments for localised skin cancers, but often cause scarring and can potentially cause serious injury and disfigurement. Consequently, escharotic salves are very strictly regulated in most western countries and while some prescription medicines are available with this effect, unauthorized sales are illegal. Some prosecutions have been pursued over unlicensed sales of escharotic products such as Cansema.

Bismuthia is a rare dermatological condition that results from the prolonged use of bismuth.

Much more rarely than with silver, bismuth may produce a generalized persistent skin discoloration resembling argyria. This is known as bismuthia. The conjunctivae and oral mucosa, as well as skin, are commonly afflicted by the condition. Pigment granules are evenly scattered through the dermis, producing a blue, or bluish gray skin color by the scattering phenomenon. It is extremenly rare, and as a consequence is poorly understood by modern medicine. Special stains and spectroscopic methods must be utilized for identification of skin pigments affected by bismuth.

Usually, bismuth produces a black line, due to bismuth sulfide, along the gums, near the teeth, similar to a lead line. This is not likely to occur if the gum is edentulous or the gum and the teeth normal. It is more common when gingivitis and carious teeth are present. Bismuth excess may cause stomatitis. On even rarer occasions, bismuth has been reported to cause pigmentation of the vagina and cervix.

Bed bug bites are caused by bed bugs primarily of two species "Cimex lectularius" (the common bed bug) and "Cimex hemipterus". Infestation is rarely due to a lack of hygiene. These insects feed exclusively on blood and may survive a year without eating. They are attracted by body warmth and carbon dioxide. Transfer to new places is usually in the personal effects of the human they feed upon.

Dwellings can become infested with bed bugs in a variety of ways, such as:

- Bugs and eggs inadvertently brought in from other infested dwellings on a visiting person's clothing or luggage;

- Infested items (such as furniture especially beds or couches, clothing, or backpacks) brought in a home or business;

- Nearby dwellings or infested items, if easy routes are available for travel, e.g. through ducts or false ceilings;

- Wild animals (such as bats or birds) that may also harbour bed bugs or related species such as the bat bug;

- People visiting an infested area (e.g. dwelling, means of transport, entertainment venue, or lodging) and carrying the bugs to another area on their clothing, luggage, or bodies. Bedbugs are increasingly found in air travel.

- Though bed bugs will feed on pets, they do not live or travel on the skin of their hosts, and pets are not believed a factor in their spread.

Razor burn is a less serious condition caused by shaving, characterized by mild to moderate redness and irritation on the surface of the skin. Unlike PFB, it is usually transient and there is no infection involved.

There is also a condition called "folliculitis barbae". The difference between the two is the cause of the inflammation in the hair follicles. Folliculitis barbae is caused by viral or bacterial infections, where pseudofolliculitis is caused by irritation from shaving and ingrown hairs.

A related condition, pseudofolliculitis nuchae, occurs on the back of the neck, often along the posterior hairline, when curved hairs are cut short and allowed to grow back into the skin. Left untreated, this can develop into acne keloidalis nuchae, a condition where hard, dark keloid-like bumps form on the neck. Both occur frequently in black men in the military, where it is so common that services often have widely known protocols for management

With extra care taken to the health of the shrimp, it is possible to prevent cases of black gill disease. The water should have 10-20 parts per thousand parts salinity and filtered.

Placing the cat's water in a shallow dish may prevent the chin from absorbing the bacteria in the water while the cat is drinking. If the cat is allergic to plastics or dyes, using a stainless-steel or glass dish is recommended . Cats may also have food allergies that make the development of acne more likely, so that switching kibble, or changing to a hydrolysed diet may be effective. Maintaining good hygiene and grooming habits make the development of feline acne less likely. Washing and exfoliating the chin with a gentle benzoyl-peroxide solution also may be preventive of further outbreaks.

The exact symptoms of a chemical burn depend on the chemical involved. Symptoms include itching, bleaching or darkening of skin, burning sensations, trouble breathing, coughing blood and/or tissue necrosis. Common sources of chemical burns include sulfuric acid (HSO), hydrochloric acid (HCl), sodium hydroxide (NaOH), lime (CaO), silver nitrate (AgNO), and hydrogen peroxide (HO). Effects depend on the substance; hydrogen peroxide removes a bleached layer of skin, while nitric acid causes a characteristic color change to yellow in the skin, and silver nitrate produces noticeable black stains. Chemical burns may occur through direct contact on body surfaces, including skin and eyes, via inhalation, and/or by ingestion. Lipophilic substances that diffuse efficiently in human tissue, e.g., hydrofluoric acid, sulfur mustard, and dimethyl sulfate, may not react immediately, but instead produce the burns and inflammation hours after the contact. Chemical fabrication, mining, medicine, and related professional fields are examples of occupations where chemical burns may occur. Hydrofluoric acid leaches into the bloodstream and reacts with calcium and magnesium, and the resulting salts can cause cardiac arrest after eating through skin.

Tinea nigra (also known as "superficial phaeohyphomycosis," and "Tinea nigra palmaris et plantaris") is a superficial fungal infection that causes dark brown to black painless patches on the palms of the hands and the soles of the feet.

This infection is caused by the fungus formerly classified as "Exophiala werneckii", but more recently classified as "Hortaea werneckii". The causative organism has also been described as "Phaeoannellomyces werneckii".

In Belgium, the Conseil Supérieur de la Santé gives a scientific advisory report on public health policy, the Superior Health Council of Belgium provides an overview of products that are authorized in Belgium for consumer use and that contain caustic substances, as well as of the risks linked to exposure to these products. This report aims at suggesting protection measures for the consumers, and formulates recommendations that apply to the different stages of the chain, which begins with the formulation of the product, followed by its regulation / marketing / application and post-application and ends with its monitoring.

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.

Exogenous ochronosis can be caused from long-term usage of certain "skin lightening" products, even if the hydroquinone is in amounts as small as 2%.

Skin lightening products are still prevalent in many parts of the world. This may be due to aesthetic or social standing reasons, in areas where a lighter skin tone is considered to be a sign of wealth or beauty.

As well, skin-lightening creams containing compounds such as hydroquinone are commonly used to help with hyperpigmentation disorders such as melasma.

Hydroquinone is the compound most frequently used in skin whitening products. Due to concerns about its side effects, it was almost banned by the FDA in 2006, as there were medical issues of carcinogenicity and reports of disfiguring ochronosis. In the European Union hydroquinone has been banned in cosmetic creams since 2000.

Long-term use of creams containing this compound may lead to exogenous ochronotic lesions. The duration of the use is directly proportional to the risk of developing the condition with most cases being after years of use.

Around 10–15 million skin lightening products are sold annually, with Japan being the major buyer.

A number of treatments have been found to be effective. A 2012 review of different treatments for skin warts in otherwise healthy people concluded modest benefit from salicylic acid and cryotherapy appears similar to salicylic acid.

Bed bugs occur around the world. Rates of infestations in developed countries, while decreasing from the 1930s to the 1980s, have increased dramatically since the 1980s. Previous to this they were common in the developing world but rare in the developed world. The increase in the developed world may have been caused by increased international travel, resistance to insecticides, and the use of new pest-control methods that do not affect bed bugs. The fall in bed bug populations after the 1930s in the developed world is believed to be partly due to the usage of DDT to kill cockroaches. The invention of the vacuum cleaner and simplification of furniture design may have also played a role. Others believe it might simply be the cyclical nature of the organism.

Tinea capitis caused by species of "Microsporum" and "Trichophyton" is a contagious disease that is endemic in many countries. Affecting primarily pre-pubertal children between 6 and 10 years, it is more common in males than females; rarely does the disease persist past age sixteen. Because spread is thought to occur through direct contact with afflicted individuals, large outbreaks have been known to occur in schools and other places where children are in close quarters; however, indirect spread through contamination with infected objects ("fomites") may also be a factor in the spread of infection. In the USA, tinea capitis is thought to occur in 3-8% of the pediatric population; up to one-third of households with contact with an infected person may harbor the disease without showing any symptoms.

The fungal species responsible for causing tinea capitis vary according to the geographical region, and may also change over time. For example, "Microsporum audouinii" was the predominant etiological agent in North America and Europe until the 1950s, but now "Trichophyton tonsurans" is more common in the USA, and becoming more common in Europe and the United Kingdom. This shift is thought to be due to the widespread use of griseofulvin, which is more effective against "M. audounii" than "T. tonsurans"; also, changes in immigration patterns and increases in international travel have likely spread "T. tonsurans" to new areas. Another fungal species that has increased in prevalence is "Trichophyton violaceum", especially in urban populations of the United Kingdom and Europe.

Treatment is predominantly preventive. Avoidance of topical phenols and diets low in tyrosine may help. Replacement and repair of damaged tissue is also possible.

There are multiple sources known to cause black gill disease. Poor pond conditions can cause debris to build up in the gills turning them black. Certain kinds of bacteria and the fungus genus Fusarium are also known causes.

Pseudofolliculitis barbae (PFB), also known as barber's itch, folliculitis barbae traumatica, razor bumps, scarring pseudofolliculitis of the beard, and shave bumps, is a medical term for persistent irritation caused by shaving. Pseudofolliculitis barbae was first described in 1956.

A recent retrospective study of all cases of Ecthyma gangrenosum from 2004-2010 in a university hospital in Mexico shows that neutropenia in immunocompromised patients is the most common risk factor for ecthyma gangrenosum.

Actinic elastosis, also known as solar elastosis, is an accumulation of abnormal elastin (elastic tissue) in the dermis of the skin, or in the conjunctiva of the eye, which occurs as a result of the cumulative effects of prolonged and excessive sun exposure, a process known as photoaging.

Leukoedema is common. It occurs in about 70-90% of black skinned adults and about 50% of black skinned children. The prevalence in white skinned people is considerably less, but reports range from less than 10% to more than 90%, probably varying depending upon the population studied, and the methods used in the study, e.g. examination conditions and the diagnostic criteria. The ethnic variation may be explained by genetic factors or simply because black skinned people have greater amount of melanin in the mucosa, making it appear darker (termed racial or physiologic pigmentation). This darker mucosa may make the edematous changes more noticeable, whereas in the mucosa of people with lighter skin types leukoedema gives a milder presentation.