Infection first presents with severe abdominal pain, nausea, vomiting, and weakness, which gradually lessens and progresses to fever, and then to CNS symptoms and severe headache and stiffness of the neck.

CNS symptoms begin with mild cognitive impairment and slowed reactions, and in a very severe form often progress to unconsciousness. Patients may present with neuropathic pain early in the infection. Eventually severe infection will lead to ascending weakness, quadriparesis, areflexia, respiratory failure, and muscle atrophy, and will lead to death if not treated. Occasionally patients present with cranial nerve palsies, usually in nerves 7 and 8, and rarely larvae will enter ocular structures. Even with treatment, damage to the CNS may be permanent and result in a variety of negative outcomes depending on the location of the infection, and the patient may suffer chronic pain as a result of infection.

Although organisms such as bacteria function as parasites, the usage of the term "parasitic disease" is usually more restricted. The three main types of organisms causing these conditions are protozoa (causing protozoan infection), helminths (helminthiasis), and ectoparasites. Protozoa and helminths are usually endoparasites (usually living inside the body of the host), while ectoparasites usually live on the surface of the host. Occasionally the definition of "parasitic disease" is restricted to diseases due to endoparasites.

Light infestations (<100 worms) frequently have no symptoms. Heavier infestations, especially in small children, can present gastrointestinal problems including abdominal pain and distension, bloody or mucus-filled diarrhea, and tenesmus (feeling of incomplete defecation, generally accompanied by involuntary straining). Mechanical damage to the intestinal mucosa may occur, as well as toxic or inflammatory damage to the intestines of the host. While appendicitis may be brought on by damage and edema of the adjacent tissue, if there are large numbers of worms or larvae present, it has been suggested that the embedding of the worms into the ileocecal region may also make the host susceptible to bacterial infection. A severe infection with high numbers of embedded worms in the rectum leads to edema, which can cause rectal prolapse, although this is typically only seen in small children. The prolapsed, inflamed and edematous rectal tissue may even show visible worms.

Growth retardation, weight loss, nutritional deficiencies, and anemia (due to long-standing blood loss) are also characteristic of infection, and these symptoms are more prevalent and severe in children. It does not commonly cause eosinophilia.

Coinfection of "T. trichiura" with other parasites is common and with larger worm burdens can cause both exacerbation of dangerous trichuriasis symptoms such as massive gastrointestinal bleeding (shown to be especially dramatic with coinfection with "Salmonella typhi") and exacerbation of symptoms and pathogenesis of the other parasitic infection (as is typical with coinfection with "Schistosoma mansoni", in which higher worm burden and liver egg burden is common). Parasitic coinfection with HIV/AIDS, tuberculosis, and malaria is also common, especially in Sub-saharan Africa, and helminth coinfection adversely affects the natural history and progression of HIV/AIDS, tuberculosis, and malaria and can increase clinical malaria severity. In a study performed in Senegal, infections of soil-transmitted helminths like "T. trichiura" (as well as schistosome infections independently) showed enhanced risk and increased the incidence of malaria.

Heavy infestations may have bloody diarrhea. Long-standing blood loss may lead to iron-deficiency anemia. Vitamin A deficiency may also result due to infection.

Physiological reactions to "Toxocara" infection depend on the host’s immune response and the parasitic load. Most cases of "Toxocara" infection are asymptomatic, especially in adults. When symptoms do occur, they are the result of migration of second stage "Toxocara" larvae through the body.

Covert toxocariasis is the least serious of the three syndromes and is believed to be due to chronic exposure. Signs and symptoms of covert toxocariasis are coughing, fever, abdominal pain, headaches, and changes in behavior and ability to sleep. Upon medical examination, wheezing, hepatomegaly, and lymphadenitis are often noted.

High parasitic loads or repeated infection can lead to visceral larva migrans (VLM). VLM is primarily diagnosed in young children, because they are more prone to exposure and ingestion of infective eggs. "Toxocara" infection commonly resolves itself within weeks, but chronic eosinophilia may result. In VLM, larvae migration incites inflammation of internal organs and sometimes the central nervous system. Symptoms depend on the organ(s) affected. Patients can present with pallor, fatigue, weight loss, anorexia, fever, headache, rash, cough, asthma, chest tightness, increased irritability, abdominal pain, nausea, and vomiting. Sometimes the subcutaneous migration tracks of the larvae can be seen. Patients are commonly diagnosed with pneumonia, bronchospasms, chronic pulmonary inflammation, hypereosinophilia, hepatomegaly, hypergammaglobulinaemia (IgM, IgG, and IgE classes), leucocytosis, and elevated anti-A and –B isohaemagglutinins. Severe cases have occurred in people who are hypersensitive to allergens; in rare cases, epilepsy, inflammation of the heart, pleural effusion, respiratory failure, and death have resulted from VLM.

Ocular larva migrans (OLM) is rare compared with VLM. A light "Toxocara" burden is thought to induce a low immune response, allowing a larva to enter the host’s eye. Although there have been cases of concurrent OLM and VLM, these are extremely exceptional. OLM often occurs in just one eye and from a single larva migrating into and encysting within the orbit. Loss of vision occurs over days or weeks. Other signs and symptoms are red eye, white pupil, fixed pupil, retinal fibrosis, retinal detachment, inflammation of the eye tissues, retinal granulomas, and strabismus. Ocular granulomas resulting from OLM are frequently misdiagnosed as retinoblastomas. "Toxocara" damage in the eye is permanent and can result in blindness.

A case study published in 2008 supported the hypothesis that eosinophilic cellulitis may also be caused by infection with "Toxocara". In this study, the adult patient presented with eosinophilic cellulitis, hepatosplenomegaly, anemia, and a positive ELISA for "T. cani"s.

Symptoms of parasites may not always be obvious. However, such symptoms may mimic anemia or a hormone deficiency. Some of the symptoms caused by several worm infestation can include itching affecting the anus or the vaginal area, abdominal pain, weight loss, increased appetite, bowel obstructions, diarrhea, and vomiting eventually leading to dehydration, sleeping problems, worms present in the vomit or stools, anemia, aching muscles or joints, general malaise, allergies, fatigue, nervousness. Symptoms may also be confused with pneumonia or food poisoning.

The effects caused by parasitic diseases range from mild discomfort to death.

The nematode parasites "Necator americanus" and "Ancylostoma duodenale" cause human hookworm infection, which leads to anaemia and protein malnutrition. This infection affects approximately 740 million people in the developing countries, including children and adults, of the tropics specifically in poor rural areas located in sub-Saharan Africa, Latin America, South-East Asia and China.

Chronic hookworm in children leads to impaired physical and intellectual development, school performance and attendance are reduced.

Pregnant women affected by a hookworm infection can also develop aneamia, which results in negative outcomes both for the mother and the infant. Some of them are: low birth weight, impaired milk production, as well as increased risk of death for the mother and the baby.

The incubation period for "Toxocara canis" and "cati" eggs depends on temperature and humidity. "T. canis" females, specifically, are capable of producing up to 200,000 eggs a day that require 2-6 weeks minimum up to a couple months before full development into the infectious stage. Under ideal summer conditions, eggs can mature to the infective stage after two weeks outside of a host. Provided sufficient oxygen and moisture availability, "Toxocara" eggs can remain infectious for years, as their resistant outer shell enables the protection from most environmental threats.However, as identified in a case study presented within the journal of helminthology, the second stage of larvae development poses strict vulnerabilities to certain environmental elements. High temperatures and low moisture levels will quickly degrade the larvae during this stage of growth.

Eustrongylidosis is a parasitic disease that mainly affects wading birds worldwide; however, the parasite’s complex, indirect life cycle involves other species such as aquatic worms and fish. Moreover, this disease is zoonotic which means the parasite can transmit disease from animals to humans. Eustrongylidosis is named after the causative agent Eustrongylides and typically occurs in eutrophicated waters where concentrations of nutrients and minerals are high enough to provide ideal conditions for the parasite to thrive and persist. Because eutrophication has become a common issue due to agricultural runoff and urban development, cases of Eustrongylidosis are becoming prevalent and hard to control. Eustrongylidosis can be diagnosed before or after death by observing behavior, clinical signs and performing fecal flotations and necropsies. Methods to control Eustrongylidosis include preventing eutrophication and providing hosts with uninfected food sources in aquaculture farms. Parasites are known to be indicators of environmental health and stability and should therefore be studied further to better understand the parasite’s life cycle and how it affects predator-prey interactions and improve conservation efforts.

Symptoms becomes evident only when the intensity of infection is relatively high. Thus the degree of negative outcomes is directly related to worm burden; more worms means greater severity of disease.

A few days after ingestion epigastric pain, fever, vomiting, and loss of appetite resulting from migration of larvae through intestinal wall to the abdominal cavity will appear in the patient. Migration in the subcutaneous tissues (under the skin) causes intermittent, migratory, painful, pruritic swellings (cutaneous larva migrans). Patches of edema appear after the above symptoms clear and are usually found on the abdomen. These lesions vary in size and can be accompanied by pruritus, rash, and stabbing pain. Swellings may last for 1 to 4 weeks in a given area and then reappear in a different location. Migration to other tissues (visceral larva migrans), can result in cough, hematuria, ocular (eye) involvement, meningitis, encephalitis and eosinophilia. Eosinophilic myeloencephalitis may also result from invasion of the central nervous system by the larvae.

Most conditions of STH have a light worm burden and usually have no discernible symptoms. Heavy infections however cause a range of health problems, including abdominal pain, diarrhoea, blood and protein loss, rectal prolapse, and physical and mental retardation.

Severe ascariasis is typically a pneumonia, as the larvae invades lungs, producing fever, cough and dyspnoea during early stage of infection.

Hookworm infections insinuate a skin reaction (dermatitis), increased white blood cells (eosinophils), a pulmonary reaction (pneumonitis), and skin rash (urticarial).

Iron deficiency anaemia due to blood loss is a common symptom.

Trichuriasis, also known as whipworm infection, is an infection by the parasitic worm "Trichuris trichiura" (whipworm). If infection is only with a few worms, there are often no symptoms. In those who are infected with many worms, there may be abdominal pain, tiredness and diarrhea. The diarrhea sometimes contains blood. Infections in children may cause poor intellectual and physical development. Low red blood cell levels may occur due to loss of blood.

The disease is usually spread when people eat food or drink water that contains the eggs of these worms. This may occur when contaminated vegetables are not fully cleaned or cooked. Often these eggs are in the soil in areas where people defecate outside and where untreated human feces is used as fertilizer. These eggs originate from the feces of infected people. Young children playing in such soil and putting their hands in their mouths also become infected easily. The worms live in the large bowel and are about four centimetres in length. Whipworm is diagnosed by seeing the eggs when examining the stool with a microscope. Eggs are barrel-shaped. Trichuriasis belongs to the group of soil-transmitted helminthiases.

Prevention is by properly cooking food and hand washing before cooking. Other measures include improving access to sanitation such as ensuring use of functional and clean toilets and access to clean water. In areas of the world where the infections are common, often entire groups of people will be treated all at once and on a regular basis. Treatment is with three days of the medication: albendazole, mebendazole or ivermectin. People often become infected again after treatment.

Whipworm infection affected about 464 million in 2015. It is most common in tropical countries. In the developing world, those infected with whipworm often also have hookworms and ascariasis infections. They have a large effect on the economy of many countries. Work is ongoing to develop a vaccine against the disease. Trichuriasis is classified as a neglected tropical disease.

Lymphadenitis, the swelling of the lymph nodes, is a commonly recognized symptom of many diseases. An early manifestation of filariasis, lymphadenitis more frequently occurs in the inguinal area during "B. malayi" infection and can occur before the worms mature.

The most common symptom is coughing and other typical symptoms are wheezing and weight loss. These symptoms are caused by larvae that reside in the lungs where immunity develops and the accumulation of mucus cause blockage of the airway into the lungs.

The signs and symptoms of helminthiasis depend on a number of factors including: the site of the infestation within the body; the type of worm involved; the number of worms and their volume; the type of damage the infesting worms cause; and, the immunological response of the body. Where the burden of parasites in the body is light, there may be no symptoms.

Certain worms may cause particular constellations of symptoms. For instance, taeniasis can lead to seizures due to neurocysticercosis.

In animal and human hosts, infestation by "Thelazia" may be asymptomatic, though it frequently causes watery eyes (epiphora), conjunctivitis, corneal opacity, or corneal ulcers (ulcerative keratitis). Infested humans have also reported "foreign body sensation"the feeling that something is in the eye.

Diagnosis involves simply examining the eyes and nearby tissues for the worms. Adult "Thelazia" are very active, one author described "T. californiensis" as a "short lively piece of nylon fishing line about 10 mm long."

If an animal is suspected of lungworm infection, there are many ways to detect this parasitic infection such as performing one or more of the following techniques: a complete medical history including lung auscultation (stethoscope examination), doing a chest xray, fecal examination for detection of ova or larvae, examination of respiratory secretions for ova or larvae, and/or a complete blood count (CBC) to check for signs of increase in eosinophils

"B. malayi" is one of the causative agents of lymphatic filariasis, a condition marked by infection and swelling of the lymphatic system. The disease is primarily caused by the presence of worms in the lymphatic vessels and the resulting host response. Signs of infection are typically consistent with those seen in bancroftian filariasis—fever, lymphadenitis, lymphangitis, lymphedema, and secondary bacterial infection—with a few exceptions.

In extreme cases of intestinal infestation, the mass and volume of the worms may cause the outer layers of the intestinal wall, such as the muscular layer, to tear. This may lead to peritonitis, volvulus, and gangrene of the intestine.

Strongyloides infection occurs in five forms. On acquiring the infection, there may be respiratory symptoms (Löffler's syndrome). The infection may then become chronic with mainly digestive symptoms. On reinfection (when larvae migrate through the body), there may be respiratory, skin and digestive symptoms. Finally, the hyperinfection syndrome causes symptoms in many organ systems, including the central nervous system.

The following are commonly used terms when referring to gnathostomiasis

- Gnathostoma

- Larva migrans profundus

- Nodular migratory eosinophilic panniculitis

- Physaloptera

- Spiruroid larva migrans

- Wandering swelling

- Yangtze edema

Avian malaria is most notably caused by Plasmodium relictum, a protist that infects birds in all parts of the world apart from Antarctica. There are several other species of "Plasmodium" that infect birds, such as "Plasmodium anasum" and "Plasmodium gallinaceum", but these are of less importance except, in occasional cases, for the poultry industry. The disease is found worldwide, with important exceptions. Usually, it does not kill birds. However, in areas where avian malaria is newly introduced, such as the islands of Hawaiʻi, it can be devastating to birds that have lost evolutionary resistance over time.

Infections by "Mansonella perstans", while often asymptomatic, can be associated with angioedema, pruritus, fever, headaches, arthralgias, and neurologic manifestations. "Mansonella streptocerca" can manifest on the skin via pruritus, papular eruptions and pigmentation changes. "Mansonella ozzardi" can cause symptoms that include arthralgias, headaches, fever, pulmonary symptoms, adenopathy, hepatomegaly, and pruritus. Eosinophilia is often prominent in all cases of Mansonelliasis. "M. perstans" can also present with Calabar-like swellings, hives, and a condition known as Kampala, or Ugandan eye worm. This occurs when adult M. perstans invades the conjunctiva or periorbital connective tissues in the eye. "M. perstans" can also present with hydrocele in South America. However, it is often hard to distinguish between the symptoms of Mansonelliasis and other nematode infections endemic to the same areas.

Avian malaria is a parasitic disease of birds, caused by parasite species belonging to the genera "Plasmodium" and "Hemoproteus" (phylum Apicomplexa, class Haemosporidia, family Plasmoiidae). The disease is transmitted by a dipteran vector including mosquitoes in the case of "Plasmodium" parasites and biting midges for "Hemoproteus." The range of symptoms and effects of the parasite on its bird hosts is very wide, from asymptomatic cases to drastic population declines due to the disease, as is the case of the Hawaiian honeycreepers. The diversity of parasites is large, as it is estimated that there are approximately as many parasites as there are species of hosts. Co-speciation and host switching events have contributed to the broad range of hosts that these parasites can infect, causing avian malaria to be a widespread global disease, found everywhere except Antarctica.

Anisakis is a genus of parasitic nematodes, which have lifecycles involving fish and marine mammals. They are infective to humans and cause anisakiasis. People who produce immunoglobulin E in response to this parasite may subsequently have an allergic reaction, including anaphylaxis, after eating fish infected with "Anisakis" species.