Spiders do not feed on humans and typically bites occur as a defense mechanism. This can occur from unintentional contact or trapping of the spider. Most spiders have fangs too small to penetrate human skin. Most bites by species large enough for their bites to be noticeable will have no serious medical consequences.

Medically significant spider venoms include various combinations and concentrations of necrotic agents, neurotoxins, and pharmacologically active compounds such as serotonin. Worldwide only two spider venoms have impact on humans—those of the widow and recluse spiders. Unlike snake and scorpion envenomation, widow and recluse species bites rarely have fatal consequences. However, isolated spider families have a lethal neurotoxic venom: the wandering spider in Brazil and the funnel web in Australia. However, due to limited contact between these spiders and humans, deaths have always been rare, and since the introduction of anti-venom in Australia, there have been no funnel web related deaths.

Estimating the number of spider bites that occur is difficult as the spider involvement may not be confirmed or identified. Several researchers recommend only evaluating verified bites: those that have an eyewitness to the bite, the spider is brought in, and identified by expert. With "suspected arachnidism" the diagnosis came without a spider positively identified.

AC is caused by chronic and excessive exposure to ultraviolet radiation in sunlight.

Risk factors include:

- Outdoor lifestyle: e.g. farmers, sailors, fishermen, windsurfers, mountaineers, golfers, etc. This has given rise to synonyms for this condition such as "sailor's lip" and "farmer's lip". The prevalence in agricultural workers in a semi-arid region of Brazil is reported to be 16.7%.

- Light skin complexion: the condition typically affects individuals with lighter skin tones, particularly Caucasians living in tropical regions. In one report, 96% of persons with AC had phenotype II according to the Fitzpatrick scale.

- Age: AC typically affects older individuals, and rarely those under the age of 45.

- Gender: the condition affects males more commonly than females. Sometimes this ratio is reported as high as 10:1.

Additional factors may also play a role, including tobacco use, lip irritation, poor oral hygiene, and ill-fitting dentures.

To prevent AC from developing, protective measures could be undertaken such as avoiding mid-day sun, or use of a broad-brimmed hat, lip balm with anti UVA and UVB ingredients (e.g. para-aminobenzoic acid), or sun blocking agents (e.g. zinc oxide, titanium oxide) prior to sun exposure.

No specific treatment for CTF is yet available. The first action is make sure the tick is fully removed from the skin, then acetaminophen and analgesics can be used to help relieve the fever and pain. Aspirin is not recommended for children, as it has been linked to Reye’s syndrome in some viral illnesses. Salicylates should not be used because of thrombocytopenia, and the rare occurrence of bleeding disorders. People who suspect they have been bitten by a tick or are starting to show signs of CTF should contact their physicians immediately.

Colorado tick fever is acquired by tick bite. No evidence of natural person-to-person transmission has been found. However, rare cases of transmission from blood transfusions have been reported. The virus which causes Colorado tick fever may stay in the blood for as long as four months after onset of the illness.

First signs or symptoms can occur about three to six days after the initial tick bite, although it can have incubation periods of up to 20 days. Patients usually experience a two-staged fever and illness which can continue for three days, diminish, and then return for another episode of one to three days. The virus has the ability to live in the blood stream for up to 120 days; therefore, coming in contact without proper precautions and the donation of blood are prohibited.

Initial symptoms include fever, chills, headaches, pain behind the eyes, light sensitivity, muscle pain, generalized malaise, abdominal pain, hepatosplenomegaly, nausea and vomiting, and a flat or pimply rash. During the second phase of the virus, a high fever can return with an increase in symptoms. CTF can be very severe in cases involving children and can even require hospitalization. Complications with this disease have included aseptic meningitis, encephalitis, and hemorrhagic fever, but these are rare.

CTF is seasonal, mostly occurring in the Rocky Mountain region of the United States and usually in altitudes from 4,000 to 10,000 feet (1.600 to 3.000 meters). Patients with CTF are mostly campers and young males, who most likely have been bitten because of their activities.

Prevention includes avoiding exposure to the sun and wearing sun block on the affected area.

- Cover up: wear long sleeves, slacks, and a wide-brimmed hat whenever harsh exposure is probable

- Avoid chemicals that may trigger a reaction

- Wear sunscreen at least factor 30 with a high UVA protection level

- Wear gloves and/or remain indoors after handling fruits or plants which increase sensitivity to light

Other causes of hair loss include:

- Alopecia mucinosa

- Biotinidase deficiency

- Chronic inflammation

- Diabetes

- Lupus erythematosus

- Pseudopelade of Brocq

- Telogen effluvium

- Tufted folliculitis

Many medications and conditions can cause sun sensitivity, including:

- Sulfa used in some drugs, among them some antibiotics, diuretics, COX-2 inhibitors, and diabetes drugs.

- Psoralens, coal tars, photo-active dyes (eosin, acridine orange)

- Musk ambrette, methylcoumarin, lemon oil (may be present in fragrances)

- PABA (found in sunscreens)

- Oxybenzone (UVA and UVB chemical blocker also in sunscreens)

- Salicylanilide (found in industrial cleaners)

- St John's Wort, used to treat clinical depression

- Hexachlorophene (found in some ℞ antibacterial soaps)

- Contact with sap from Giant Hogweed. Common Rue (Ruta graveolens) is another phototoxic plant commonly found in gardens. Phototoxicity caused by plants is called phytophotodermatitis.

- Tetracycline antibiotics (e.g., tetracycline, doxycycline, minocycline)

- Benzoyl peroxide

- Retinoids (e.g., isotretinoin)

- Some NSAIDs (e.g., ibuprofen, naproxen sodium)

- Fluoroquinolone antibiotic: Sparfloxacin in 2% of cases

- Amiodarone, used to treat atrial fibrillation

- Pellagra

Photo dermatitis can also be caused by plants like the Dictamnus (commonly known as the "Burning Bush") which is a genus of the flowering plant in the Rutaceae family. This is called phytophotodermatitis.

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.

For early cases, prompt treatment is usually curative. However, the severity and treatment of Lyme disease may be complicated due to late diagnosis, failure of antibiotic treatment, and simultaneous infection with other tick-borne diseases (coinfections), including ehrlichiosis, babesiosis, and immune suppression in the patient.

It is believed that less than 5% of people have lingering symptoms of fatigue, pain, or joint and muscle aches at the time they finish treatment. These symptoms can last for more than 6 months. This condition is called post-treatment lyme disease syndrome. As of 2016 the reason for the lingering symptoms was not known; the condition is generally managed similarly to fibromyalgia or chronic fatigue syndrome.

In dogs, a serious long-term prognosis may result in glomerular disease, which is a category of kidney damage that may cause chronic kidney disease. Dogs may also experience chronic joint disease if the disease is left untreated. However, the majority of cases of Lyme disease in dogs result in a complete recovery with, and sometimes without, treatment with antibiotics. In rare cases, Lyme disease can be fatal to both humans and dogs.

In dermatologic pathology, a dermal cylindroma, also dermal eccrine cylindroma or Cutaneous Cylindroma) and (less specifically) cylindroma, is a benign adnexal tumor, which occurs on the scalp and forehead.when multiple Cylindroma-Show Hat like configration.

Multiple cylindromas may grow together in a "hat-like" configuration, sometimes referred to as a turban tumor. Cylindromas are uncommon dysplasias of skin appendages.

Internationally, the prevalence rates of WKS are relatively standard, being anywhere between zero and two percent. Despite this, specific sub-populations seem to have higher prevalence rates including people who are homeless, older individuals (especially those living alone or in isolation), and psychiatric inpatients. Additionally, studies show that prevalence is not connected to alcohol consumption per capita. For example, in France, a country that is well known for its consumption and production of wine, prevalence was only 0.4% in 1994, while Australia had a prevalence of 2.8%.

In terms of frequency, is estimated at 2 per 100,000, it has identified in different regions of the world. Some clusters of certain types of autosomal dominant cerebellar ataxia reach a prevalence of 5 per 100,000.

Lyme disease is caused by spirochetal bacteria from the genus "Borrelia". Spirochetes are surrounded by peptidoglycan and flagella, along with an outer membrane similar to other Gram-negative bacteria. Because of their double-membrane envelope, "Borrelia" bacteria are often mistakenly described as Gram negative despite the considerable differences in their envelope components from Gram-negative bacteria. The Lyme-related "Borrelia" species are collectively known as "Borrelia burgdorferi sensu lato", and show a great deal of genetic diversity.

"B. burgdorferi sensu lato" is made up of 21 closely related species, but only three clearly cause Lyme disease: "B. burgdorferi sensu stricto" (predominant in North America, but also present in Europe), "B. afzelii", and "B. garinii" (both predominant in Eurasia). Some studies have also proposed "B. bissettii" and "B. valaisiana" may sometimes infect humans, but these species do not seem to be important causes of disease.

As it is associated with excessive sun or wind exposure, wearing protective sunglasses with side shields and/or wide brimmed hats and using artificial tears throughout the day may help prevent their formation or stop further growth. Surfers and other water-sport athletes should wear eye protection that blocks 100% of the UV rays from the water, as is often used by snow-sport athletes. Many of those who are at greatest risk of pterygium from work or play sun exposure do not understand the importance of protection.

The fact that gastrointestinal surgery can lead to the development of WKS was demonstrated in a study that was completed on three patients who recently undergone a gastrectomy. These patients had developed WKS but were not alcoholics and had never suffered from dietary deprivation. WKS developed between 2 and 20 years after the surgery. There were small dietary changes that contributed to the development of WKS but overall the lack of absorption of thiamine from the gastrointestinal tract was the cause. Therefore, it must be ensured that patients who have undergone gastrectomy have a proper education on dietary habits, and carefully monitor their thiamine intake. Additionally, an early diagnosis of WKS, should it develop, is very important.

A pterygium is a pinkish, triangular tissue growth on the cornea of the eye. It typically starts on the cornea near the nose. It may slowly grow but rarely grows so large that the pupil is covered. Often both eyes are involved.

The cause is unclear. It appears to be partly related to long term exposure to UV light and dust. Genetic factors also appear to be involved. It is a benign growth. Other conditions that can look similar include a pinguecula, tumor, or Terrien's marginal corneal degeneration.

Prevention may include wearing sunglasses and a hat if in an area with strong sunlight. Among those with the condition, an eye lubricant can help with symptoms. Surgical removal is typically only recommended if the ability to see is affected. Following surgery a pterygium may recur in around half of cases.

The frequency of the condition varies from 1% to 33% in various regions of the world. It occurs more commonly among males than females and in people who live closer to the equator. The condition becomes more common with age. The condition has been described since at least 1000 BC.

Dermal cylindromas are:

- dermal lesions consisting of nests of cells that are surrounded by hyaline (i.e. glassy, eosinophilic, acellular) material and have:

- hyperchromatic nuclei that may palisade (columnar nuclei arranged around the periphery of the cells nests - with their short axis tangential to the nest periphery), and

- cells with lighter staining ovoid nuclei at their centre.

They lack of a significant number of lymphocytes; this differentiates them from spiradenomas.

Opioid use disorder can develop as a result of self-medication, though this is controversial. Scoring systems have been derived to assess the likelihood of opiate addiction in chronic pain patients.

According to position papers on the treatment of opioid dependence published by the United Nations Office on Drugs and Crime and the World Health Organization, care providers should not treat opioid use disorder as the result of a weak character or will. Additionally, detoxification alone does not constitute adequate treatment.

In terms of the genetics of autosomal dominant cerebellar ataxia 11 of 18 known genes are caused by repeated expansions in corresponding proteins, sharing the same mutational mechanism. SCAs can be caused by conventional mutations or large rearrangements in genes that make glutamate and calcium signaling, channel function, tau regulation and mitochondrial activity or RNA alteration.

The mechanism of Type I is not completely known, however Whaley, et al. suggest the polyglutamine product is toxic to the cell at a protein level, this effect may be done by transcriptional dysregulation and disruption of calcium homeostasis which causes apoptosis to occur earlier.

Palinopsia from cerebrovascular accidents generally resolves spontaneously, and treatment should be focused on the vasculopathic risk factors. Palinopsia from neoplasms, AVMs, or abscesses require treatment of the underlying condition, which usually also resolves the palinopsia. Palinopsia due to seizures generally resolves after correcting the primary disturbance and/or treating the seizures. In persistent hallucinatory palinopsia, a trial of an anti-epileptic drug can be attempted. Anti-epileptics reduce cortical excitability and could potentially treat palinopsia caused by cortical deafferentation or cortical irritation. Patients with idiopathic hallucinatory palinopsia should have close follow-up.

As of 2010 it caused around 9,000 deaths, down from 34,000 in 1990. As of 2000, the disability-adjusted life-years (9 to 10 years) lost due to sleeping sickness are 2.0 million. From 2010-2014, there was an estimated 55 million people at risk for "gambiense" African Trypanosomiasis and over 6 million people at risk for "rhodesiense" African Trypanosomiasis. In 2014, the World Health Organization reported 3,797 cases of Human African Trypanosomiasis when the predicted number of cases were to be 5,000. The number of total reported cases in 2014 is an 86% reduction to the total number of cases reported in 2000.

The disease has been recorded as occurring in 37 countries, all in sub-Saharan Africa. It occurs regularly in southeast Uganda and western Kenya, and killed more than 48,000 Africans in 2008. The Democratic Republic of the Congo is the most affected country in the world, accounting for 75% of the "Trypanosoma brucei gambiense" cases. The population at risk being about 69 million with one third of this number being at a 'very high' to 'moderate' risk and the remaining two thirds at a 'low' to 'very low' risk. The number of people being affected by the disease has declined. At this rate, sleeping sickness elimination is a possibility. The World Health Organization plans to eradicate sleeping sickness by the year 2020.

"Trypanosoma brucei gambiense" accounts for the majority of African trypanosomiasis cases, with humans as the main reservoir needed for the transmission, while "Trypanosoma brucei rhodesiense" is mainly zoonotic, with the occasional human infection. African Trypanosomiasis is dependent on the interaction of the parasite (trypanosome) with the tsetse flies (vector), as well as the host (human for "Trypanosoma brucei gambiense", and animals for "Trypanosoma brucei rhodesiense"). The risk of contracting African Trypanosomiasis is dependent on coming in contact with an infected tsetse fly.

Of the published cases of palinopsia from posterior cortical lesions or seizures, 93% described hallucinatory palinopsia. Hallucinatory palinopsia may be caused by many types of posterior cortical lesions such as neoplasms, infarctions, hemorrhages, arteriovenous malformations, aneurysm, abscesses, and tuberculomas. Hallucinatory palinopsia from seizures may be secondary to a focal cortical lesion or may be secondary to a non-structural disturbance. Causes of seizures that are reported to cause palinopsia include metabolic disturbances (hyperglycemia, carnitine deficiency), ion channel disturbances, Creutzfeldt–Jakob disease, and seizures of unknown cause.