Genetic, environmental, hormonal, and immune-system factors have been shown to be involved in the manifestation of seborrhoeic dermatitis.

Seborrhoeic dermatitis may be aggravated by illness, psychological stress, fatigue, sleep deprivation, change of season and reduced general health.

In children, excessive vitamin A intake can cause seborrhoeic dermatitis. Lack of biotin, pyridoxine (vitamin B), or riboflavin (vitamin B) may be a cause in babies.

Those with immunodeficiency (especially infection with HIV) and with neurological disorders such as Parkinson's disease (for which the condition is an autonomic sign) and stroke are particularly prone to it.

The interaction between fecal enzyme activity and IDD explains the observation that infant diet and diaper rash are linked because fecal enzymes are in turn affected by diet. Breast-fed babies, for example, have a lower incidence of diaper rash, possibly because their stools have higher pH and lower enzymatic activity. Diaper rash is also most likely to be diagnosed in infants 8–12 months old, perhaps in response to an increase in eating solid foods and dietary changes around that age that affect fecal composition. Any time an infant’s diet undergoes a significant change (i.e. from breast milk to formula or from milk to solids) there appears to be an increased likelihood of diaper rash.

The link between feces and IDD is also apparent in the observation that infants are more susceptible to developing diaper rash after treating with antibiotics, which affect the intestinal microflora. Also, there is an increased incidence of diaper rash in infants who have suffered from diarrhea in the previous 48 hours, which may be because fecal enzymes such as lipase and protease are more active in feces which have passed rapidly through the gastrointestinal tract.

If the condition thickens, turns red and irritated, starts spreading, appears on other body parts, or if the baby develops thrush (fungal mouth infection), fungal ear infection (an ear infection that does not respond to antibiotics) or a persistent diaper rash, medical intervention is recommended.

Severe cases of cradle cap, especially with cracked or bleeding skin, can provide a place for bacteria to grow. If the cradle cap is caused by a fungal infection which has worsened significantly over days or weeks to allow bacterial growth (impetigo, most commonly), a combination treatment of antibiotics and antifungals may be necessary. Since it is difficult for a layperson to distinguish the difference between sebaceous gland cradle cap, fungal cradle cap, or either of these combined with a bacterial infection, medical advice should be sought if the condition appears to worsen.

Cradle cap is occasionally linked to immune disorders. If the baby is not thriving and has other problems (e.g. diarrhea), a doctor should be consulted.

Although wetness alone has the effect of macerating the skin, softening the stratum corneum, and greatly increasing susceptibility to friction injury, urine has an additional impact on skin integrity because of its effect on skin pH. While studies show that ammonia alone is only a mild skin irritant, when urea breaks down in the presence of fecal urease it increases pH because ammonia is released, which in turn promotes the activity of fecal enzymes such as protease and lipase. These fecal enzymes increase the skin's hydration and permeability to bile salts which also act as skin irritants.

There is no detectable difference in rates of diaper rash in conventional disposable diaper wearers and reusable cloth diaper wearers. "Babies wearing superabsorbent disposable diapers with a central gelling material have fewer episodes of diaper dermatitis compared with their counterparts wearing cloth diapers. However, keep in mind that superabsorbent diapers contain dyes that were suspected to cause allergic contact dermatitis (ACD)." Whether wearing cloth or disposable diapers they should be changed frequently to prevent diaper rash, even if they don't feel wet. To reduce the incidence of diaper rash, disposable diapers have been engineered to pull moisture away from the baby's skin using synthetic non-biodegradable gel. Today, cloth diapers use newly available superabsorbent microfiber cloth placed in a pocket with a layer of light permeable material that contacts the skin. This design serves to pull moisture away from the skin in to the microfiber cloth. This technology is used in most major pocket cloth diapers brands today.

Assurances that this condition will clear as the baby matures are very common. However, studies have shown that the condition occasionally persists into the toddler years, and less commonly into later childhood. It tends to recur in adolescence and persists into adulthood. In an Australian study, about 15 percent of previously diagnosed children still had eczema 10 years later. Sometimes, cradle cap turns into atopic dermatitis. Rarely, it turns out to be misdiagnosed psoriasis.

Seborrhoeic dermatitis may involve an inflammatory reaction to a proliferation of a form of the yeast "Malassezia", though this has not been proven.

The main species of yeast found on the scalp of those with the condition is "Malassezia globosa", others being "Malassezia furfur" (formerly known as "Pityrosporum ovale") and "Malassezia restricta". It has been suggested that the yeast produces toxic substances that irritate and inflame the skin. Patients with seborrhoeic dermatitis appear to have a reduced resistance to the yeast. However, the colonization rate of affected skin may be lower than that of unaffected skin.

Only saturated fatty acids (FAs) have been shown to support "Malassezia" growth. It has also been shown that while number density of "M. globosa" and "M. restricta" do not directly correlate to dandruff presence or severity, removal correlates directly with amelioration of flaking. Furthermore, in dandruff-susceptible individuals pure oleic acid, an unsaturated FA and "Malassezia" metabolite, induces flaking in the absence of "Malassezia" by direct effects on the host skin barrier. These findings support the following hypothesis:

"Malassezia" hydrolyze human sebum, releasing a mixture of saturated and unsaturated fatty acids. They take up the required saturated FAs, leaving behind unsaturated FAs. The unsaturated FAs penetrate the stratum corneum. Because of their non-uniform structure, they breach the skin's barrier function. This barrier breach induces an irritation response, leading to dandruff and seborrhoeic dermatitis.

There is no good evidence that a mother's diet during pregnancy, the formula used, or breastfeeding changes the risk. There is tentative evidence that probiotics in infancy may reduce rates but it is insufficient to recommend its use.

People with eczema should not get the smallpox vaccination due to risk of developing eczema vaccinatum, a potentially severe and sometimes fatal complication.

Most cases are well managed with topical treatments and ultraviolet light. About 2% of cases are not. In more than 60% of young children, the condition subsides by adolescence.

Research is looking into connections between hair loss and other health issues. While there has been speculation about a connection between early-onset male pattern hair loss and heart disease, a review of articles from 1954 to 1999 found no conclusive connection between baldness and coronary artery disease. The dermatologists who conducted the review suggested further study was needed.

Environmental factors are under review. A 2007 study indicated that smoking may be a factor associated with age-related hair loss among Asian men. The study controlled for age and family history, and found statistically significant positive associations between moderate or severe male pattern hairloss and smoking status.

Vertex baldness is associated with an increased risk of coronary heart disease (CHD) and the relationship depends upon the severity of baldness, while frontal baldness is not. Thus, vertex baldness might be a marker of CHD and is more closely associated with atherosclerosis than frontal baldness.

Other causes of hair loss include:

- Alopecia mucinosa

- Biotinidase deficiency

- Chronic inflammation

- Diabetes

- Lupus erythematosus

- Pseudopelade of Brocq

- Telogen effluvium

- Tufted folliculitis

The number of cases of human louse infestations (or pediculosis) has increased worldwide since the mid-1960s, reaching hundreds of millions annually. It is estimated between 1 and 20% of specific groups in Europe are infected.

Despite improvements in medical treatment and prevention of human diseases during the 20th century, head louse infestation remains stubbornly prevalent. In 1997, 80% of American elementary schools reported at least one outbreak of lice. Lice infestation during that same period was more prevalent than chickenpox.

About 6–12 million children between the ages of 3 and 11 are treated annually for head lice in the United States alone. High levels of louse infestations have also been reported from all over the world, including Israel, Denmark, Sweden, U.K., France, and Australia.

The number of children per family, the sharing of beds and closets, hair washing habits, local customs and social contacts, healthcare in a particular area (e.g. school), and socioeconomic status were found to be significant factors in head louse infestation. Children between 4 and 13 years of age are the most frequently infested group. In the U.S., African-American children have lower rates of infestation.

The United Kingdom's National Health Service and many American health agencies report that lice "prefer" clean hair because it's easier to attach eggs and to cling to the strands; however, this is often contested.

Head lice ("Pediculus humanus capitis") infestation is most frequent on children aged 3–10 and their families. Females get head lice twice as often as males, and infestation in persons of or other black descent is rare because of hair consistency. But these children may have nits that hatch and the live lice could be transferred by head contact to other children.

Head lice are generally spread through direct head-to-head contact with an infested person. Transmission by sharing bedding or clothing such as headwear is much less common. The cause of head lice infestations is not related to cleanliness. Neither hair length nor how often the hair is brushed affect the risk of infection.

Body lice are spread through direct contact with the body, clothing, or other personal items of a person already carrying lice. Pubic lice are most often spread by intimate contact with an infested person. Head lice occur on the head hair, body lice on the clothing, and pubic lice mainly on the hair near the groin. Lice cannot burrow into the skin.

Other lice that infest humans are the body louse and the crab louse. The claws of these three species are adapted to attachment to specific hair diameters.

Food allergies develop more easily in people with the atopic syndrome, a very common combination of diseases: allergic rhinitis and conjunctivitis, eczema, and asthma. The syndrome has a strong inherited component; a family history of allergic diseases can be indicative of the atopic syndrome.

An Institute of Medicine report says that food proteins contained in vaccines, such as gelatin, milk, or egg can cause sensitization (development of allergy) in vaccine recipients, to those food items.

Hypersensitivity pneumonitis may also be called many different names, based on the provoking antigen. These include:

Of these types, Farmer's Lung and Bird-Breeder's Lung are the most common. "Studies document 8-540 cases per 100,000 persons per year for farmers and 6000-21,000 cases per 100,000 persons per year for pigeon breeders. High attack rates are documented in sporadic outbreaks. Prevalence varies by region, climate, and farming practices. HP affects 0.4–7% of the farming population. Reported prevalence among bird fanciers is estimated to be 20-20,000 cases per 100,000 persons at risk."

The best treatment is to avoid the provoking allergen, as chronic exposure can cause permanent damage. Corticosteroids such as prednisolone may help to control symptoms but may produce side-effects.

Reduced salivary flow rate is associated with increased caries since the buffering capability of saliva is not present to counterbalance the acidic environment created by certain foods. As a result, medical conditions that reduce the amount of saliva produced by salivary glands, in particular the submandibular gland and parotid gland, are likely to lead to dry mouth and thus to widespread tooth decay. Examples include Sjögren's syndrome, diabetes mellitus, diabetes insipidus, and sarcoidosis. Medications, such as antihistamines and antidepressants, can also impair salivary flow. Stimulants, most notoriously methylamphetamine, also occlude the flow of saliva to an extreme degree. This is known as meth mouth. Tetrahydrocannabinol (THC), the active chemical substance in cannabis, also causes a nearly complete occlusion of salivation, known in colloquial terms as "cotton mouth". Moreover, 63% of the most commonly prescribed medications in the United States list dry mouth as a known side-effect. Radiation therapy of the head and neck may also damage the cells in salivary glands, somewhat increasing the likelihood of caries formation.

Susceptibility to caries can be related to altered metabolism in the tooth, in particular to fluid flow in the dentin. Experiments on rats have shown that a high-sucrose, cariogenic diet "significantly suppresses the rate of fluid motion" in dentin.

The use of tobacco may also increase the risk for caries formation. Some brands of smokeless tobacco contain high sugar content, increasing susceptibility to caries. Tobacco use is a significant risk factor for periodontal disease, which can cause the gingiva to recede. As the gingiva loses attachment to the teeth due to gingival recession, the root surface becomes more visible in the mouth. If this occurs, root caries is a concern since the cementum covering the roots of teeth is more easily demineralized by acids than enamel. Currently, there is not enough evidence to support a causal relationship between smoking and coronal caries, but evidence does suggest a relationship between smoking and root-surface caries.

Exposure of children to secondhand tobacco smoke is associated with tooth decay.

Intrauterine and neonatal lead exposure promote tooth decay. Besides lead, all atoms with electrical charge and ionic radius similar to bivalent calcium,

such as cadmium, mimic the calcium ion and therefore exposure to them may promote tooth decay.

Poverty is also a significant social determinant for oral health. Dental caries have been linked with lower socio-economic status and can be considered a disease of poverty.

Forms are available for risk assessment for caries when treating dental cases; this system using the evidence-based Caries Management by Risk Assessment (CAMBRA). It is still unknown if the identification of high-risk individuals can lead to more effective long-term patient management that prevents caries initiation and arrests or reverses the progression of lesions.

Saliva also contains iodine and EGF. EGF results effective in cellular proliferation, differentiation and survival. Salivary EGF, which seems also regulated by dietary inorganic iodine, plays an important physiological role in the maintenance of oral (and gastro-oesophageal) tissue integrity, and, on the other hand, iodine is effective in prevention of dental caries and oral health.

The majority of patients present in their mid-30s to late 40s. This is likely due to a combination of the slow growth of the bone and the decreased participation in activities associated with surfer's ear past the 30's. However surfer's ear is possible at any age and is directly proportional to the amount of time spent in cold, wet, windy weather without adequate protection.

The normal ear canal is approximately 7mm in diameter and has a volume of approximately 0.8 ml (approximately one-sixth of a teaspoon). As the condition progresses the diameter narrows and can even close completely if untreated, although sufferers generally seek help once the passage has constricted to 0.5-2mm due to the noticeable hearing impairment. While not necessarily harmful in and of itself, constriction of the ear canal from these growths can trap debris, leading to painful and difficult to treat infections.

Factors that can cause male as well as female infertility are:

- DNA damage

- DNA damage reduces fertility in female ovocytes, as caused by smoking, other xenobiotic DNA damaging agents (such as radiation or chemotherapy) or accumulation of the oxidative DNA damage 8-hydroxy-deoxyguanosine

- DNA damage reduces fertility in male sperm, as caused by oxidative DNA damage, smoking, other xenobiotic DNA damaging agents (such as drugs or chemotherapy) or other DNA damaging agents including reactive oxygen species, fever or high testicular temperature

- General factors

- Diabetes mellitus, thyroid disorders, undiagnosed and untreated coeliac disease, adrenal disease

- Hypothalamic-pituitary factors

- Hyperprolactinemia

- Hypopituitarism

- The presence of anti-thyroid antibodies is associated with an increased risk of unexplained subfertility with an odds ratio of 1.5 and 95% confidence interval of 1.1–2.0.

- Environmental factors

- Toxins such as glues, volatile organic solvents or silicones, physical agents, chemical dusts, and pesticides. Tobacco smokers are 60% more likely to be infertile than non-smokers.

German scientists have reported that a virus called Adeno-associated virus might have a role in male infertility, though it is otherwise not harmful. Other diseases such as chlamydia, and gonorrhea can also cause infertility, due to internal scarring (fallopian tube obstruction).

The widespread use of wetsuits has allowed people to surf in much colder waters, which has increased the incidence and severity of surfer's ear for people who do not properly protect their ears.

- Avoid activity during extremely cold or windy conditions.

- Keep the ear canal as warm and dry as possible.

- Ear plugs

- Wetsuit hood

- Swim cap

- Diving helmet

There are certain diseases and disorders affecting teeth that may leave an individual at a greater risk for cavities.

Molar incisor hypomineralization, which seems to be increasingly common. While the cause is unknown it is thought to be a combination of genetic and environmental factors. Possible contributing factors that have been investigated include systemic factors such as high levels of dioxins or polychlorinated biphenyl (PCB) in the mother’s milk, premature birth and oxygen deprivation at birth, and certain disorders during the child’s first 3 years such as such as mumps, diphtheria, scarlet fever, measles, hypoparathyroidism, malnutrition, malabsorption, hypovitaminosis D, chronic respiratory diseases, or undiagnosed and untreated coeliac disease, which usually presents with mild or absent gastrointestinal symptoms.

Amelogenesis imperfecta, which occurs in between 1 in 718 and 1 in 14,000 individuals, is a disease in which the enamel does not fully form or forms in insufficient amounts and can fall off a tooth. In both cases, teeth may be left more vulnerable to decay because the enamel is not able to protect the tooth.

In most people, disorders or diseases affecting teeth are not the primary cause of dental caries. Approximately 96% of tooth enamel is composed of minerals. These minerals, especially hydroxyapatite, will become soluble when exposed to acidic environments. Enamel begins to demineralize at a pH of 5.5. Dentin and cementum are more susceptible to caries than enamel because they have lower mineral content. Thus, when root surfaces of teeth are exposed from gingival recession or periodontal disease, caries can develop more readily. Even in a healthy oral environment, however, the tooth is susceptible to dental caries.

The evidence for linking malocclusion and/or crowding to dental caries is weak; however, the anatomy of teeth may affect the likelihood of caries formation. Where the deep developmental grooves of teeth are more numerous and exaggerated, pit and fissure caries is more likely to develop (see next section). Also, caries is more likely to develop when food is trapped between teeth.

Researchers commonly base demographic studies on infertility prevalence on a five-year period. Practical measurement problems, however, exist for any definition, because it is difficult to measure continuous exposure to the risk of pregnancy over a period of years.

These differ according to the type of chronic liver disease.

- Excessive alcohol use

- Obesity

- Metabolic syndrome including raised blood lipids

- Health care professionals who are exposed to body fluids and infected blood

- Sharing infected needle and syringes

- Having unprotected sex and multiple sex partners

- Working with toxic chemicals without wearing safety clothes

- Certain prescription medications