In intestinal schistosomiasis, eggs become lodged in the intestinal wall and cause an immune system reaction called a granulomatous reaction. This immune response can lead to obstruction of the colon and blood loss. The infected individual may have what appears to be a potbelly. Eggs can also become lodged in the liver, leading to high blood pressure through the liver, enlarged spleen, the buildup of fluid in the abdomen, and potentially life-threatening dilations or swollen areas in the esophagus or gastrointestinal tract that can tear and bleed profusely (esophageal varices). In rare instances, the central nervous system is affected. Individuals with chronic active schistosomiasis may not complain of typical symptoms.

The first potential reaction is an itchy, papular rash that results from cercariae penetrating the skin, often in a person's first infection. The round bumps are usually one to three centimeters across. Because people living in affected areas have often been repeatedly exposed, acute reactions are more common in tourists and migrants. The rash can occur between the first few hours and a week after exposure and lasts for several days. A similar, more severe reaction called "swimmer's itch" reaction can also be caused by cercariae from animal trematodes that often infect birds.

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.

Clinical presentation of sparganosis most often occurs after the larvae have migrated to a subcutaneous location. The destination of the larvae is often a tissue or muscle in the chest, abdominal wall, extremities, or scrotum, although other sites include the eyes, brain, urinary tract, pleura, pericardium, and spinal canal. The early stages of disease in humans are often asymptomatic, but the spargana typically cause a painful inflammatory reaction in the tissues surrounding the subcutaneous site as they grow. Discrete subcutaneous nodules develop that may appear and disappear over a period of time. The nodules usually itch, swell, turn red, and migrate, and are often accompanied by painful edema. Seizures, hemiparesis, and headaches are also common symptoms of sparganosis, especially cerebral sparganosis, and eosinophilia is a common sign. Clinical symptoms also vary according to the location of the sparganum; possible symptoms include elephantiasis from location in the lymph channels, peritonitis from location in the intestinal perforation, and brain abscesses from location in the brain. In genital sparganosis, subcutaneous nodules are present in the groin, labia, or scrotum and may appear tumor-like.

Ocular sparganosis a particularly well-described type of sparganosis. Early signs of the ocular form include eye pain, epiphora (excessive watering of the eye), and/or ptosis (drooping of the upper eyelid). Other signs include periorbital edema and/or edematous swelling that resembles Romana’s sign in Chagas disease, lacrimation, orbital cellulitis, exophthalmos (protrusion of the eyeball), and/or an exposed cornea ulcer. The most common sign at presentation is a mass lesion in the eye. If untreated, ocular sparganosis can lead to blindness.

In one case of brain infestation by "Spirometra erinaceieuropaei", a man sought treatment on suffering headaches, seizures, memory flashbacks and strange smells. Magnetic resonance imaging (MRI) scans showed a cluster of rings, initially in the right medial temporal lobe, but moving over time to the other side of the brain. The cause was not determined for four years; ultimately a biopsy was performed and a 1 cm-long tapeworm was found and removed. The patient continued to suffer symptoms.

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 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.

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.

The incubation period ranges from 4 days to approximately 8 weeks. The infection leads to significant weight loss and anaemia. Various symptoms are observed, including fever, oedema, adenitis, dermatitis and nervous disorders. The disease cannot be diagnosed with certainty except physically detecting parasites by blood microscopic examination or various serological reactions.

Symptoms of opisthorchiasis (caused by "Opisthorchis" spp.) are indistinguishable from clonorchiasis (caused by "Clonorchis sinensis"). About 80% of infected people have no symptoms, though they can have eosinophilia. Asymptomatic infection can occur when there are less than 1000 eggs in one gram in feces. Infection is considered heavy when there are 10,000-30,000 eggs in one gram of feces. Symptoms of heavier infections with "Opisthorchis viverrini" may include: diarrhoea, epigastric and upper right quadrant pain, lack of appetite (anorexia), fatigue, yellowing of the eyes and skin (jaundice) and mild fever.

These parasites are long-lived and cause heavy chronic infections that may lead to accumulation of fluid in the legs (edema) and in the peritoneal cavity (ascites), enlarged non-functional gall-bladder and also cholangitis, which can lead to periductal fibrosis, cholecystitis and cholelithiasis, obstructive jaundice, hepatomegaly and/or fibrosis of the periportal system.

Animal trypanosomiasis, also known as nagana and nagana pest, or sleeping sickness, is a disease of vertebrates. The disease is caused by trypanosomes of several species in the genus "Trypanosoma" such as "Trypanosoma brucei". "Trypanosoma vivax" causes nagana mainly in West Africa, although it has spread to South America. The trypanosomes infect the blood of the vertebrate host, causing fever, weakness, and lethargy, which lead to weight loss and anemia; in some animals the disease is fatal unless treated. The trypanosomes are transmitted by tsetse flies.

An interesting feature is the remarkable tolerance to nagana pathology shown by some breeds of cattle, notably the N'Dama – a West African "Bos taurus" breed. This contrasts with the susceptibility shown by East African "Bos indicus" cattle such as the zebu.

Sparganosis is typically diagnosed following surgical removal of the worms, although the infection may also be diagnosed by identification of eosinophilia or identification of the parasite in a tissue specimen. If such biopsy and excision procedures are not feasible, the antisparganum ELISA test may be used. In theory, a pre-operative diagnosis could be made by identification of exposure history and a painful, migratory, subcutaneous nodule. Sparganosis usually presents as a single nodule, while other cestode infections such as cysticercosis typically present as multiple nodules. Preoperative diagnosis, however, is rare.

CT and MRI scans are especially useful for diagnosis of cerebral sparganosis, as they reveal lesions in the brain. Through a retrospective analysis of 25 cases of cerebral sparganosis from 2000 to 2006, Song et al. found a number of characteristic signs that could be used in the future to diagnose cerebral sparganosis without performing an excision or tissue biopsy. The most characteristic finding was the "tunnel sign" on MRI images, showing the migrating track of the worm, while the most common finding was multiple conglomerated ring-shaped enhancements, seen as bead-shaped, usually with 3 to 6 rings. These findings led Song et al. to suggest that clinical history, ELISA, and either MRI or CT scans could be sufficient to make a sparganosis diagnosis. These lesions, however, are sometimes mistaken for tuberculosis lesions. In one case cerebral sparganosis was not diagnosed for four years, during which scans showed a cluster of rings moving from the right to the left side of the brain; ultimately the worm was found on biopsy.

Opisthorchiasis is a parasitic disease caused by species in the genus Opisthorchis (specifically, "Opisthorchis viverrini" and "Opisthorchis felineus"). Chronic infection may lead to cholangiocarcinoma, a malignant cancer of the bile ducts.

Medical care and loss of wages caused by "Opisthorchis viverrini" in Laos and in Thailand costs about $120 million annually. Infection by "Opisthorchis viverrini" and other liver flukes in Asia affect the poor and poorest people. Opisthorchiasis is on the World Health Organization's list of neglected tropical diseases.

A persistent or recurrent cough that gets aggravated at night, weakness, weight loss and a low fever raises the possible diagnosis of this disease. Some children with this disease may also have enlarged lymph nodes in the neck and elsewhere. Others may cough up a little blood and may also have a wheeze.

Neglected tropical diseases (NTDs) are a diverse group of tropical infections which are especially common in low-income populations in developing regions of Africa, Asia, and the Americas. They are caused by a variety of pathogens such as viruses, bacteria, protozoa and helminths. These diseases are contrasted with the big three diseases (HIV/AIDS, tuberculosis, and malaria), which generally receive greater treatment and research funding. In sub-Saharan Africa, the effect of these diseases as a group is comparable to malaria and tuberculosis. NTD co-infection can also make HIV/AIDS and tuberculosis more deadly.

In some cases, the treatments are relatively inexpensive. For example, the treatment for schistosomiasis is US$0.20 per child per year. Nevertheless, in 2010 it was estimated that control of neglected diseases would require funding of between US$2 billion and US$3 billion over the subsequent five to seven years. Some pharmaceutical companies have committed to donating all the drug therapies required, and mass drug administration (for example mass deworming) has been successfully accomplished in several countries. However, preventive measures are often more accessible in the developed world, but not universally available in poorer areas.

Within developed countries, neglected tropical diseases affect the very poorest in society. In the United States, there are up to 1.46 million families including 2.8 million children living on less than two dollars a day. In countries such as these, the burdens of neglected tropical diseases are often overshadowed by other public health issues. However, many of the same issues put populations at risk in developed as developing nations. For example, from poverty stem problems such as lack of adequate housing, thus exposing individuals to the vectors of these diseases.

Twenty neglected tropical diseases are prioritized by the World Health Organization (WHO), though other organizations define NTDs differently. Chromoblastomycosis and other deep mycoses, scabies and other ectoparasites and snakebite envenoming were added to the list in 2017. These diseases are common in 149 countries, affecting more than 1.4 billion people (including more than 500 million children) and costing developing economies billions of dollars every year. They resulted in 142,000 deaths in 2013—down from 204,000 deaths in 1990. Of these 20, two were targeted for eradication (dracunculiasis (guinea-worm disease) by 2015 and yaws by 2020), and four for elimination (blinding trachoma, human African trypanosomiasis, leprosy and lymphatic filariasis by 2020).

Parasitic pneumonia is an infection of the lungs by parasites. It is a rare cause of pneumonia, occurring almost exclusively in immunocompromised persons (persons with a weakened or absent immune system). This is a respiratory infection that may or may not be serious.

There are a variety of parasites which can affect the lungs. In general, these parasites enter the body through the skin or by being swallowed. Once inside the body, these parasites travel to the lungs, most often through the blood. There, a similar combination of cellular destruction and immune response causes disruption of oxygen transportation. Depending on the type of parasite, antihelmynthic drugs can be prescribed.

The most common parasites involved:

- Ascariasis

- Schistosoma

- Toxoplasma gondii

Lymphatic filariasis is also known as elephantiasis. There are approximately 120 million individuals infected and 40 million with deformities. Approximately two-thirds of cases are in Southwest Asia and one-third in Africa. Lymphatic filariasis is rarely fatal. Lymphatic filariasis has lifelong implications, such as lymphoedema of the limbs, genital disease, and painful recurrent attacks. Most people are asymptomatic, but have lymphatic damage. Up to 40 percent of infected individuals have kidney damage. It is a vector-borne disease, caused by nematode worms that are transmitted by mosquitoes.

It can be treated with cost-effective antihelminthic treatments, and washing skin can slow or even reverse damage. It is diagnosed with a finger-prick blood test.

Tropical (pulmonary) eosinophilia, or TPE, is characterized by coughing, asthmatic attacks, and an enlarged spleen, and is caused by "Wuchereria bancrofti", a filarial infection. It occurs most frequently in India and Southeast Asia. Tropical eosinophilia is considered a manifestation of a species of microfilaria. This disease can be confused with tuberculosis, asthma, or coughs related to roundworms.

Tropical pulmonary eosinophilia is a rare, but well recognised, syndrome characterised by pulmonary interstitial infiltrates and marked peripheral eosinophilia. This condition is more widely recognised and promptly diagnosed in filariasis-endemic regions, such as the Indian subcontinent, Africa, Asia and South America. In nonendemic countries, patients are commonly thought to have bronchial asthma. Chronic symptoms may delay the diagnosis by up to five years. Early recognition and treatment with the antifilarial drug, diethylcarbamazine, is important, as delay before treatment may lead to progressive interstitial fibrosis and irreversible impairment.

The condition of marked eosinophilia with pulmonary involvement was first termed tropical pulmonary eosinophilia in 1950. The syndrome is caused by a distinct hypersensitive immunological reaction to microfilariae of" W. bancrofti" and "Brugia malayi". However, only a small percentage (< 0.5%) of the 130 million people globally who are infected with filariasis apparently develop this reaction. The clearance of rapidly opsonised microfilariae from the bloodstream results in a hypersensitive immunological process and abnormal recruitment of eosinophils, as reflected by extremely high IgE levels of over 1000 kU/L. The typical patient is a young adult man from the Indian subcontinent.

Swimmer’s itch or cercarial dermatitis, is a short-term immune reaction occurring in the skin of humans that have been infected by water-borne schistosomatidae. Symptoms, which include itchy, raised papules, commonly occur within hours of infection and do not generally last more than a week. It is common in freshwater, brackish and marine habitats worldwide. Incidence may be on the rise, although this may also be attributed to better monitoring. Nevertheless, the condition has been regarded as emerging infectious disease.

There are no permanent effects to people from this condition. Orally administered hydroxyzine, an antihistamine, is sometimes prescribed to treat swimmer's itch and similar dermal allergic reactions. In addition, bathing in oatmeal, baking soda, or Epsom salts can also provide relief of symptoms.

Swimmer’s itch probably has been around as long as humans. The condition was known to exist as early as the 1800s, but it was not until 1928 that a biologist found that the dermatitis was caused by the larval stage of a group of flatworm parasites in the family Schistosomatidae. The genera most commonly associated with swimmer’s itch in humans are Trichobilharzia and Gigantobilharzia. It can also be caused by schistosome parasites of non-avian vertebrates, such as "Schistosomatium douthitti", which infects snails and rodents. Other taxa reported to cause the reaction include "Bilharziella polonica" and "Schistosoma bovis". In marine habitats, especially along the coasts, swimmer’s itch can occur as well.

These parasites use both freshwater snails and vertebrates as hosts in their parasitic life cycles as follows:

1. Once a schistosome egg is immersed in water, a short-lived, non-feeding, free-living stage known as the miracidium emerges. The miracidium uses cilia to follow chemical and physical cues thought to increase its chances of finding the first intermediate host in its life cycle, a freshwater snail.

2. After infecting a snail, it develops into a mother sporocyst, which in turn undergoes asexual reproduction, yielding large numbers of daughter sporocysts, which asexually produce another short-lived, free-living stage, the cercaria.

3. Cercariae use a tail-like appendage (often forked in genera causing swimmer’s itch) to swim to the surface of the water; and use various physical and chemical cue in order to locate the next and final (definitive) host in the life cycle, a bird. These larvae can accidentally come into contact with the skin of a swimmer. The cercaria penetrates the skin and dies in the skin immediately. The cercariae cannot infect humans, but they cause an inflammatory immune reaction. This reaction causes initially mildly itchy spots on the skin. Within hours, these spots become raised papules which are intensely itchy. Each papule corresponds to the penetration site of a single parasite.

4. After locating a bird, the parasite penetrates through the skin (usually the feet), dropping the forked tail in the process. Inside the circulatory system, the immature worms (schistosomula) develop into mature male and female worms, mate and migrate through the host’s circulatory system (or nervous system in case of "T. regenti") to the final location (veins feeding the gastrointestinal tract) within the host body. There they lay eggs in the small veins in the intestinal mucosa from which they make their way into the lumen of the gut, and are dumped into the water when the bird defecates. One European species, "Trichobilharzia regenti", instead infects the bird host’s nasal tissues and larvae hatch from the eggs directly in the tissue during drinking/feeding of the infected birds.

The most common symptoms include headache, muscle aches, and fatigue. A rash may occur, but is uncommon. Ehrlichiosis can also blunt the immune system by suppressing production of TNF-alpha, which may lead to opportunistic infections such as candidiasis.

Most of the signs and symptoms of ehrlichiosis can likely be ascribed to the immune dysregulation that it causes. A "toxic shock-like" syndrome is seen in some severe cases of ehrlichiosis. Some cases can present with purpura and in one such case the organisms were present in such overwhelming numbers that in 1991 Dr. Aileen Marty of the AFIP was able to demonstrate the bacteria in human tissues using standard stains, and later proved that the organisms were indeed Ehrlichia using immunoperoxidase stains.

Experiments in mouse models further supports this hypothesis, as mice lacking TNF-alpha I/II receptors are resistant to liver injury caused by ehrlichia infection.

3% of human monocytic ehrlichiosis cases result in death; however, these deaths occur "most commonly in immunosuppressed individuals who develop respiratory distress syndrome, hepatitis, or opportunistic nosocomial infections."

An opportunistic infection is an infection caused by pathogens (bacteria, viruses, fungi, or protozoa) that take advantage of an opportunity not normally available, such as a host with a weakened immune system, an altered microbiota (such as a disrupted gut flora), or breached integumentary barriers. Many of these pathogens do not cause disease in a healthy host that has a normal immune system. However, a compromised immune system, a penetrating injury, or a lack of competition from normal commensals presents an opportunity for the pathogen to infect.

Ehrlichiosis is a tickborne

bacterial infection, caused by bacteria of the family Anaplasmataceae, genera "Ehrlichia" and "Anaplasma". These obligate intracellular bacteria infect and kill white blood cells.

The average reported annual incidence is on the order of 2.3 cases per million people.

The portal of entry is the gastrointestinal tract. The organism is acquired usually by insufficiently cooked pork or contaminated water, meat, or milk. Acute "Y. enterocolitica" infections usually lead to mild self-limiting enterocolitis or terminal ileitis and adenitis in humans. Symptoms may include watery or bloody diarrhea and fever, resembling appendicitis or salmonellosis or shigellosis. After oral uptake, "Yersinia" species replicate in the terminal ileum and invade Peyer's patches. From here they can disseminate further to mesenteric lymph nodes causing lymphadenopathy. This condition can be confused with appendicitis, so is called pseudoappendicitis. In immunosuppressed individuals, they can disseminate from the gut to the liver and spleen and form abscesses. Because "Yersinia" species are siderophilic (iron-loving) bacteria, people with hereditary hemochromatosis (a disease resulting in high body iron levels) are more susceptible to infection with "Yersinia" (and other siderophilic bacteria). In fact, the most common contaminant of stored blood is "Y. enterocolitica". See yersiniosis for further details.

Diagnosis is usually made by identification of the fungi from clinical specimens. Biopsies of skin lesions, lymph nodes, and bone marrow demonstrate the presence of organisms on histopathology.

The most common symptoms are fever, skin lesions, anemia, generalized lymphadenopathy, and hepatomegaly.

"Penicillium marneffei" demonstrates in vitro susceptibility to multiple antifungal agents including ketoconazole, itraconazole, miconazole, flucytosine, and amphotericin B. Without treatment patients have a poor prognosis; death occur by liver failure as the fungus releases toxins in the bloodstream. The elevation of liver enzyme in the blood helps to establish a diagnosis.