Head lice are generally uncomfortable, but typically do not constitute a serious condition. The most common symptom is itching of the head, which normally worsens 3 to 4 weeks after the initial infestation. The bite reaction is very mild, and it can be rarely seen between the hairs. Bites can be seen, especially in the neck of long-haired individuals when the hair is pushed aside. Swelling of the local lymph nodes and fever are rare. Itching may cause skin breakdown and uncommonly result in a bacterial infection.

In Ethiopia, head lice appear to be able to spread louse-born epidemic typhus and "Bartonella quintana". In Europe, the head lice do not appear to carry these infections.

Head lice infestation, also known as pediculosis capitis and nits, is the infection of the head hair and scalp by the head louse ("Pediculus humanus capitis"). Itching from lice bites is common. During a person's first infection, the itch may not develop for up to six weeks. If a person is infected again, symptoms may begin much more quickly. The itch may cause problems with sleeping. Generally, however, it is not a serious condition. While head lice appear to spread some other diseases in Africa, they do not appear to do so in Europe or North America.

Head lice are spread by direct contact with the hair of someone who is infected. The cause of head lice infestations are not related to cleanliness. Other animals, such as cats and dogs, do not play a role in transmission. Head lice feed only on human blood and are only able to survive on human head hair. When adults, they are about 2 to 3 mm long. When not attached to a human, they are unable to live beyond three days. Humans can also become infected with two other lice – the body louse and the crab louse. To make the diagnosis, live lice must be found. Using a comb can help with detection. Empty eggshells (known as nits) are not sufficient for the diagnosis.

Possible treatments include: combing the hair frequently with a fine tooth comb or shaving the head completely. A number of topical medications are also effective, including malathion, ivermectin, and dimethicone. Dimethicone, which is a silicone oil, is often preferred due to the low risk of side effects. Pyrethroids such as permethrin have been commonly used; however, have become less effective due to increasing pesticide resistance. There is little evidence for alternative medicines.

Head-lice infestations are common, especially in children. In Europe, they infect between 1 and 20% of different groups of people. In the United States, between 6 and 12 million children are infected a year. They occur more often in girls than boys. It has been suggested that historically, head lice infection were beneficial, as they protected against the more dangerous body louse. Infestations may cause stigmatization of the infected individual.

To diagnose infestation, the entire scalp should be combed thoroughly with a louse comb and the teeth of the comb should be examined for the presence of living lice after each time the comb passes through the hair. The use of a louse comb is the most effective way to detect living lice.

The most characteristic symptom of infestation is pruritus (itching) on the head which normally intensifies 3 to 4 weeks after the initial infestation. The bite reaction is very mild and it can be rarely seen between the hairs. Excessive scratching of the infested areas can cause sores, which may become infected.

Head-lice infestation is most frequent on children aged 3–10 and their families. Approximately 3% of school children in the United States contract head lice. Females are more frequently infested than males. Those of African descent rarely suffer infestation due to differences in hair texture.

Head lice are spread through direct head-to-head contact with an infested person. From each egg or "nit" may hatch one nymph that will grow and develop to the adult louse. Lice feed on blood once or more often each day by piercing the skin with their tiny needle-like mouthparts. While feeding they excrete saliva, which irritates the skin and causes itching. Lice cannot burrow into the skin.

Serious infestations and chronic attacks can cause anxiety, stress, and insomnia. Development of refractory delusional parasitosis is possible, as a person develops an overwhelming obsession with bed bugs.

Individual responses to bites vary, ranging from no visible effect (in about 20–70%), to small macular spots, to prominent wheals and bullae formations along with intense itching that may last several days. The bites often occur in a line. A central hemorrhagic spot may also occur due to the release of anticoagulants in the saliva.

Symptoms may not appear until some days after the bites have occurred. Reactions often become more brisk after multiple bites due to possible sensitization to the salivary proteins of the bed bug. The skin reaction usually occurs in the area of the bite which is most commonly the arms, shoulders and legs as they are more frequently exposed at night. Numerous bites may lead to an erythematous rash or urticaria.

In the classic scenario, the itch is made worse by warmth, and is usually experienced as being worse at night, possibly because distractions are fewer. As a symptom, it is less common in the elderly.

The symptoms of this disease include:

- Severe pruritus

- Pain

- Inflammation and swelling

- Lesions and ulcerations, with black dots in the center

Left untreated, secondary infections, such as bacteremia, tetanus, and gangrene, can occur.

In all cases, tungiasis by itself only caused morbidity, though secondary infection may lead to mortality. The life cycle section presents the Fortaleza stages from the flea’s developmental perspective. It should be noted that the discussion is specific to symptoms of human infection. The clinical presentation in humans follows the Fortaleza Classification as the stage of infection will determine the symptoms present. The following discussion will give an overview of the symptoms beginning in stage 2 because patients are not likely to present themselves at the early stages of infection, mostly because the flea’s burrowing is usually not felt. This may be due to a keratolytic enzyme secreted during stage 1.

The patient with a single flea may present as early as stage 2 when, though the erythema is barely perceptible, a boring pain and the curious sensation of pleasant itching occur. This inflammatory reaction is the initial immunological response to the infestation. Heavily infested patients may not notice a stage 2 infection due to the other fleas’ causing irritation as well. Feces may be seen, but this is more common in the 3rd stage.

Around the third day after penetration, erythema and skin tenderness are felt, accompanied by pruritus (severe itching) and a black furuncular nodule surrounded by a white halo of stretched skin caused by the expansion of the flea. Fecal coils may protrude from the center of the nodule where the flea’s anus is facing upward. They should be washed off quickly as the feces may remain in the skin unless removed. During this 3a substage, pain can be severe, especially at night or, if the nodule is on the foot, while walking. Eggs will also begin to be released and a watery secretion can be observed. The radical metamorphosis during the 3rd to 6th day after penetration, or neosomy, precedes the formation of a small caldera-like rim rampart as a result of the increased thickness of the flea’s chitin exoskeleton. During the caldera formation, the nodule shrinks a bit and it looks as if it is beginning to dry out; this takes 2 weeks and comprises substage 3b.

At the third week after penetration and substage 4a, the eggs’ release will have stopped and the lesion will become smaller and more wrinkled. As the flea is near death, fecal and water secretion will stop altogether. Pain, tenderness, and skin inflammation will still be present. Around the 25th day after penetration, the lesion looks like a black crust and the flea’s carcass is removed by host repair mechanisms and the skin begins to heal. With the flea gone, inflammation may still persist for a while.

Although patients would not present within the 5th stage of tungiasis as the flea would be dead and no longer in the body, this stage is characterized by the reorganization of the skin (1–4 weeks) and a circular residue of 5–10 mm in diameter around the site in penetration. An intraepithelial abscess, which developed due to the presence of the flea, will drain and later heal. Although these disease residues would persist for a few months, tungiasis is no longer present.

In severe cases, ulcers are common, as well as complete tissue and nail deformation. A patient may be unable to walk due to severe pain if too many of the lesions are present in the feet. Suppuration (pus formation), auto-amputation of digits (via ainhum), and chronic lymphedema may also be seen.

If the patient is not vaccinated, tetanus is often a complication due to secondary infection. Gangrene is another common complication of severe infestation and superinfection. Staphylococcus aureus and Wolbachia endobacteria can be transmitted by the chigoe flea, as well as nearly 150 other different pathogens. For these reasons, the chigoe flea should be removed as soon as possible.

The characteristic symptoms of a scabies infection include intense itching and superficial burrows. The burrow tracks are often linear, to the point that a neat "line" of four or more closely placed and equally developed mosquito-like "bites" is almost diagnostic of the disease. Because the host develops the symptoms as a reaction to the mites' presence over time, typically a delay of four to six weeks occurs between the onset of infestation and the onset of itching. Similarly, symptoms often persist for one to several weeks after successful eradication of the mites. As noted, those re-exposed to scabies after successful treatment may exhibit symptoms of the new infestation in a much shorter period—as little as one to four days.

The main symptom is itching, usually in the pubic-hair area, resulting from hypersensitivity to louse saliva, which can become stronger over two or more weeks following initial infestation. In some infestations, a characteristic grey-blue or slate coloration macule appears ("maculae caeruleae") at the feeding site, which may last for days. Nits or live lice may also be visible to the unaided eye. Adult lice can sometimes be seen crawling on the skin.

The rabbit ear mite, "Psoroptes cuniculi", is larger than "Otodectes cynotis". It causes thick firm debris to form in the ear canal, and can eventually migrate to the skin of the outer ear and face. Symptoms include scratching and shaking of the head. Treatment includes topical selamectin, or injections of ivermectin and frequent cleanings of the rabbit's environment.

Pediculosis corporis (also known as "pediculosis vestimenti" and "vagabond's disease") is a cutaneous condition caused by body lice (specifically "Pediculus corporis") that lay their eggs in the seams of clothing.

Body lice are a nuisance in themselves and cause intense itching. They are also vectors (transmitters) of other diseases and can spread epidemic typhus, trench fever, and louse-borne relapsing fever.

The ear mite is the most common cause of ear infections in cats, quickly spreading from one cat to another through direct contact. Ear mites cause inflammatory symptoms, similar to bacterial and yeast infections. Symptoms include itching and redness of the ears. Other, more serious problems can result from untreated infections, such as skin disease in areas other than the ear like the neck and tail, and deafness.

Pediculosis pubis (also known as "crabs" and "pubic lice") is a disease caused by the pubic louse, "Pthirus pubis", a parasitic insect notorious for infesting human pubic hair. The species may also live on other areas with hair, including the eyelashes, causing pediculosis ciliaris. Infestation usually leads to intense itching in the pubic area. Treatment with topic agents such as permethrin or pyrethrin with piperonyl butoxide is effective. Worldwide, pediculosis pubis affects about 2% of the population.

How myiasis affects the human body depends on where the larvae are located. Larvae may infect dead, necrotic (prematurely dying) or living tissue in various sites: the skin, eyes, ears, stomach and intestinal tract, or in genitourinary sites. They may invade open wounds and lesions or unbroken skin. Some enter the body through the nose or ears. Larvae or eggs can reach the stomach or intestines if they are swallowed with food and cause gastric or intestinal myiasis.

Several different presentations of myiasis and their symptoms:

Mange is a class of skin diseases caused by parasitic mites. Since mites also infect plants, birds, and reptiles, the term "mange", suggesting poor condition of the hairy coat due to the infection, is sometimes reserved only for pathological mite-infestation of nonhuman mammals. Thus, mange includes mite-associated skin disease in domestic animals (cats and dogs), in livestock (such as sheep scab), and in wild animals (for example, coyotes, cougars, and bears). Since mites belong to the arachnid subclass Acari (also called Acarina), another term for mite infestation is acariasis.

Parasitic mites that cause mange in mammals embed themselves either in skin or hair follicles in the animal, depending upon their genus. "Sarcoptes" spp. burrow into skin, while "Demodex" spp. live in follicles.

In humans, these two types of mite infections, which would otherwise be known as "mange" in furry mammals, are instead known respectively as scabies and demodicosis.

There are several complications with the terminology:

Acariasis is a term for a rash, caused by mites, sometimes with a papillae (pruritic dermatitis), and usually accompanied by severe itching sensations. An example of such an infection is scabies.

The closely related term, mange, is commonly used with domestic animals (pets) and also livestock and wild mammals, whenever hair-loss is involved. "Sarcoptes" and "Demodex" species are involved in mange, but both of these genera are also involved in human skin diseases (by convention only, not called mange). "Sarcoptes" in humans is especially severe symptomatically, and causes the condition scabies noted above.

Another genus of mite which causing itching but rarely causes hair loss because it burrows only at the keratin level, is "Cheyletiella." Various species of this genus of mite also affect a wide variety of mammals, including humans.

Mite infestation sometimes implies an ectoparasitic, cutaneous condition such as dermatitis. However, it is possible for mites to invade the gastrointestinal and urinary tracts.

MeSH uses the term "Mite Infestations" as pertaining to Acariformes. However, mites not in this grouping can be associated with human disease. (See "Classification", below.)

The term Acari refers to ticks and mites together, which can cause ambiguity. (Mites are a paraphyletic grouping).

Mites can be associated with disease in at least three different ways: (1) cutaneous dermatitis, (2) production of allergin, and (3) as a vector for parasitic diseases. The language used to describe mite infestation often does not distinguish among these.

Nosocomial myiasis is myiasis acquired in a hospital setting. It is quite frequent, as patients with open wounds or sores can be infested if flies are present. To prevent nosocomial myiasis, hospital rooms must be kept free of flies.

Tungiasis (also known as "nigua", "pio" and "bicho de pie", or "pique" or sand flea disease) is an inflammatory skin disease caused by infection with the female ectoparasitic "Tunga penetrans" (also known as chigoe flea, jigger, nigua or sand flea), found in the tropical parts of Africa, the Caribbean, Central and South America, and India. "Tunga penetrans" is the smallest known flea, measuring 1 mm across. It is also known in Latin America as the "nigua" and "bicho de pie" (Spanish) or "bicho de pé" (Portuguese), literally ""foot bug"". "Tunga penetrans" is a member of the genus "Tunga", which comprises 13 species.

Tungiasis causes skin inflammation, severe pain, itching, and a lesion at the site of infection that is characterized by a black dot at the center of a swollen red lesion, surrounded by what looks like a white halo. Desquamation of the skin is always seen, especially after the flea expands during hypertrophy.

As of 2009, tungiasis is present worldwide in 88 countries with varying degrees of incidence. This disease is of special public health concern in highly endemic areas such as Nigeria, Trinidad and Tobago, and Brazil, where its prevalence, especially in poor communities, has been known to approach 50%.

The chigoe flea is properly classified as a member of the order Siphonaptera as it is a flea. Although commonly referred to as chiggers, true chiggers are mites which are minute arachnids. Mites penetrate the skin and feed on skin cells that are broken down by an enzyme they secrete from their mouthparts, but they do not lay eggs in the host as "T. penetrans" does. Moreover, in mites, the adult and the larval forms both feed on other animals. This is not the case with "T. penetrans", as only the adults feed on mammals and it is only the female that stays attached to the host.

"Tunga penetrans" is also known by the following names: chigoe flea, sand flea, nigua, chigger flea, jigger flea, bicho de pé, pico, sikka, kuti, and piqui, among many others.

Infections on the body may give rise to typical enlarging raised red rings of ringworm. Infection on the skin of the feet may cause athlete's foot and in the groin, jock itch. Involvement of the nails is termed onychomycosis, and they may thicken, discolour, and finally crumble and fall off. They are common in most adult people, with up to 20% of the population having one of these infections at any given moment.

Animals such as dogs and cats can also be affected by ringworm, and the disease can be transmitted between animals and humans, making it a zoonotic disease.

Specific signs can be:

- red, scaly, itchy or raised patches

- patches may be redder on outside edges or resemble a ring

- patches that begin to ooze or develop blister

- bald patches may develop, when the scalp is affected

- nails may thicken, discolour or begin to crack

Sarcoptic mange, also known as canine scabies, is a highly contagious infestation of "Sarcoptes scabiei canis", a burrowing mite. The canine sarcoptic mite can also infest cats, pigs, horses, sheep, and various other species. The human analog of burrowing mite infection, due to a closely related species, is called scabies (the "seven year itch").

All these burrowing mites are in the family Sarcoptidae. They dig into and through the skin, causing intense itching from an allergic reaction to the mite, and crusting that can quickly become infected. Hair loss and crusting frequently appear first on elbows and ears. Skin damage can occur from the dog's intense scratching and biting. Secondary skin infection is also common. Dogs with chronic sarcoptic mange are often in poor condition, and in both animals and humans, immune suppression from starvation or any other disease causes this type of mange to develop into a highly crusted form in which the burden of mites is far higher than in healthy specimens.

Myiasis is a parasitic infestation caused by larvae of several fly species. Diagnosis and treatment are generally quite simple. This infestation is, however, rarely seen in the vulvar area. Infestation of vulvar area with larvae and maggots is called vulvar myiasis. Very few cases have been described in literature.

Flystrike in sheep is a myiasis condition, in which domestic sheep are infected by one of several species of flies which are external parasites of sheep.

An ectoparasitic infestation is a parasitic disease caused by organisms that live primarily on the surface of the host.

Examples:

- Scabies

- Crab louse (pubic lice)

- Pediculosis (head lice)

- "Lernaeocera branchialis" (cod worm)