Areas with the highest prevalence of helminthiasis are tropical and subtropical areas including sub-Saharan Africa, central and east Asia, and the Americas.

Some types of helminthiases are classified as neglected tropical diseases. They include:

- Soil-transmitted helminthiases

- Roundworm infections such as lymphatic filariasis, dracunculiasis, and onchocerciasis

- Trematode infections, such as schistosomiasis, and food-borne trematodiases, including fascioliasis, clonorchiasis, opisthorchiasis, and paragonimiasis

- Tapeworm infections such as cysticercosis, taeniasis, and echinococcosis

Latest estimates indicate that the total annual death toll which is directly attributable is as high as 135,000. The death toll due to the malnutrition link is likely to be much higher.

Infection of "T. trichiura" is most frequent in areas with tropical weather and poor sanitation practices. Trichuriasis occurs frequently in areas in which untreated human feces is used as fertilizer or where open defecation takes place. Trichuriasis infection prevalence is 50 to 80 percent in some regions of Asia (noted especially in China and Korea) and also occurs in rural areas of the southeastern United States.

Infections are widely distributed in tropical and subtropical areas, with the greatest numbers occurring in sub-Saharan Africa, the Americas, China and east Asia.

It is estimated that a third of all pregnant women in developing countries are infected with hookworm, 56% of all pregnant women in developing countries suffer from anemia, 20% of all maternal deaths are either directly or indirectly related to anemia. Numbers like this have led to an increased interest in the topic of hookworm-related anemia during pregnancy. With the understanding that chronic hookworm infection can often lead to anemia, many people are now questioning if the treatment of hookworm could effect change in severe anemia rates and thus also on maternal and child health as well. Most evidence suggests that the contribution of hookworm to maternal anemia merits that all women of child-bearing age living in endemic areas be subject to periodic anthelmintic treatment. The World Health Organization even recommends that infected pregnant women be treated after their first trimester. Regardless of these suggestions, only Madagascar, Nepal and Sri Lanka have added deworming to their antenatal care programs.

This lack of deworming of pregnant women is explained by the fact that most individuals still fear that anthelmintic treatment will result in adverse birth outcomes. But a 2006 study by Gyorkos et al. found that when comparing a group of pregnant women treated with mebendazole with a control placebo group, both illustrated rather similar rates in adverse birth outcomes. The treated group demonstrated 5.6% adverse birth outcomes, while the control group had 6.25% adverse birth outcomes. Furthermore, Larocque et al. illustrated that treatment for hookworm infection actually led to positive health results in the infant. This study concluded that treatment with mebendazole plus iron supplements during antenatal care significantly reduced the proportion of very low birth weight infants when compared to a placebo control group. Studies so far have validated recommendations to treat infected pregnant women for hookworm infection during pregnancy.

A review of effects of antihelminthics (anti-worm drugs) given in pregnancy found that there was not enough evidence to support treating pregnant women in their second or third trimesters. The women who were treated in the second trimester and the women who had no treatment showed no difference in numbers of maternal anemia, low birth weight, preterm birth or deaths of babies.

The intensity of hookworm infection as well as the species of hookworm have yet to be studied as they relate to hookworm-related anemia during pregnancy. Additionally, more research must be done in different regions of the world to see if trends noted in completed studies persist.

Infection can be avoided by proper disposal of human feces, avoiding fecal contamination of food, not eating dirt, and avoiding crops fertilized with untreated human feces. Simple and effective proper hygiene such as washing hands and food is recommended for control.

Improved facilities for feces disposal have decreased the incidence of whipworm. Handwashing before food handling, and avoiding ingestion of soil by thorough washing of food that may have been contaminated with egg-containing soil are other preventive measures. Improvement of sanitation systems, as well as improved facilities for feces disposal, have helped to limit defecation onto soil and contain potentially infectious feces from bodily contact.

A study in a Brazillian urban centre demonstrated a significant reduction in prevalence and incidence of soil-transmitted helminthiasis, including trichuriasis, following implementation of a citywide sanitation program. A 33% reduction in the prevalence of trichuriasis and a 26% reduction in the incidence of trichuriasis was found in a study performed on 890 children ages 7–14 years old within 24 different sentinel areas chosen to represent the varied environmental conditions throughout the city of Salvador, Bahia, Brazil. Control of soil fertilizers has helped eliminate the potential for contact of human fecal matter and fertilizer in the soil.

Control of this parasite should be directed against reducing the level of

environmental contamination. Treatment of heavily infected individuals is one

way to reduce the source of contamination (one study has estimated that 60% of

the total worm burden resides in less than 10% of the population). Other

obvious methods are to improve access to sanitation, e.g. toilets, but also

convincing people to maintaining them in a clean, functional state, thereby making

them conducive to use.

"A. cantonensis" and its vectors are endemic to Southeast Asia and the Pacific Basin. The infection is becoming increasingly important as globalization allows it to spread to more and more locations, and as more travelers encounter the parasites. The parasites probably travel effectively through rats traveling as stowaways on ships, and through the introduction of snail vectors outside endemic areas.

Although mostly found in Asia and the Pacific where asymptomatic infection can be as high as 88%, human cases have been reported in the Caribbean, where as much as 25% of the population may be infected. In the United States, cases have been reported in Hawaii, which is in the endemic area [5]. The infection is now endemic in wildlife and a few human cases have also been reported in areas where the parasite was not originally endemic, such as New Orleans and Egypt.

The disease has also arrived in Brazil, where there were 34 confirmed cases from 2006 to 2014, including one death. The giant African land snail, which can be a vector of the parasite, has been introduced to Brazil as an invasive species and is spreading the disease. There may be more undiagnosed cases, as Brazilian physicians are not familiar with the eosinophilic meningitis associated to angiostrongyliasis and misdiagnose it as bacterial or viral.

It is estimated that between 576 and 740 million individuals are infected with hookworm. Of these infected individuals, about 80 million are severely affected. The major cause of hookworm infection is "N. americanus" which is found in the Americas, sub-Saharan Africa, and Asia. "A. duodenale" is found in more scattered focal environments, namely Europe and the Mediterranean. Most infected individuals are concentrated in sub-Saharan Africa and East Asia/the Pacific Islands with each region having estimates of 198 million and 149 million infected individuals, respectively. Other affected regions include: South Asia (50 million), Latin America and the Caribbean (50 million), South Asia (59 million), Middle East/North Africa (10 million). A majority of these infected individuals live in poverty-stricken areas with poor sanitation. Hookworm infection is most concentrated among the world’s poorest who live on less than $2 a day.

While hookworm infection may not directly lead to mortality, its effects on morbidity demand immediate attention. When considering disability-adjusted life years (DALYs), neglected tropical diseases, including hookworm infection, rank among diarrheal diseases, ischemic heart disease, malaria, and tuberculosis as one of the most important health problems of the developing world.

It has been estimated that as many as 22.1 million DALYs have been lost due to hookworm infection. Recently, there has been increasing interest to address the public health concerns associated with hookworm infection. For example, the Bill & Melinda Gates Foundation recently donated US$34 million to fight Neglected Tropical Diseases including hookworm infection. Former US President Clinton also announced a mega-commitment at the Clinton Global Initiative (CGI) 2008 Annual Meeting to de-worm 10 million children.

Many of the numbers regarding the prevalence of hookworm infection are estimates as there is no international surveillance mechanism currently in place to determine prevalence and global distribution. Some prevalence rates have been measured through survey data in endemic regions around the world. The following are some of the most recent findings on prevalence rates in regions endemic with hookworm.

Darjeeling, Hooghly District, West Bengal, India (Pal "et al." 2007)

- 42.8% infection rate of predominantly "N. americanus" although with some "A. duodenale" infection

- Both hookworm infection load and degree of anemia in the mild range

Xiulongkan Village, Hainan Province, China (Gandhi "et al." 2001)

- 60% infection rate of predominantly "N. americanus"

- Important trends noted were that prevalence increased with age (plateau of about 41 years) and women had higher prevalence rates than men

Hoa Binh, Northwest Vietnam (Verle "et al." 2003)

- 52% of a total of 526 tested households infected

- Could not identify species, but previous studies in North Vietnam reported "N. americanus" in more than 95% of hookworm larvae

Minas Gerais, Brazil (Fleming "et al." 2006)

- 62.8% infection rate of predominantly "N. americanus"

KwaZulu-Natal, South Africa (Mabaso "et al." 2004)

- Inland areas had a prevalence rate of 9.3% of "N. americanus"

- Coastal plain areas had a prevalence rate of 62.5% of "N. americanus"

Lowndes County, Alabama, United States

- 34.5% infection rate of predominantly "N. americanus"

There have also been technological developments that may facilitate more accurate mapping of hookworm prevalence. Some researchers have begun to use geographical information systems (GIS) and remote sensing (RS) to examine helminth ecology and epidemiology. Brooker "et al." utilized this technology to create helminth distribution maps of sub-Saharan Africa. By relating satellite derived environmental data with prevalence data from school-based surveys, they were able to create detailed prevalence maps. The study focused on a wide range of helminths, but interesting conclusions about hookworm specifically were found. As compared to other helminths, hookworm is able to survive in much hotter conditions and was highly prevalent throughout the upper end of the thermal range.

Improved molecular diagnostic tools are another technological advancement that could help improve existing prevalence statistics. Recent research has focused on the development of a DNA-based tool that can be used for diagnosis of infection, specific identification of hookworm, and analysis of genetic variability in hookworm populations. Again this can serve as a major tool for different public health measures against hookworm infection. Most research regarding diagnostic tools is now focused on the creation of a rapid and cost-effective assay for the specific diagnosis of hookworm infection. Many are hopeful that its development can be achieved within the next five years.

There are many public health strategies that can drastically limit the transmission of "A. cantonensis" by limiting contact with infected vectors. Vector control may be possible, but has not been very successful in the past. Education to prevent the introduction of rats or snail vectors outside endemic areas is important to limit the spread of the disease. There are no vaccines in development for angiostrongyliasis.

Deworming treatments in infected children may have some nutritional benefit, as worms are often partially responsible for malnutrition. However, in areas where these infections are common, there is strong evidence that mass deworming campaigns do not have a positive effect on children's average nutritional status, levels of blood haemoglobin, cognitive abilities, performance at school or survival. To achieve health gains in the longer term, improvements in sanitation and hygiene behaviours are also required, together with deworming treatments.

Education, improved sanitation, and controlled disposal of human feces are critical for prevention. Nonetheless, wearing shoes in endemic areas helps reduce the prevalence of infection.

There are 21.4 million people infected with trachoma, of whom 2.2 million are partially blind and 1.2 million are blind. It is found in Africa, Asia, Central and South America, Middle East, and Australia. The disease disproportionately affects women and children. The mortality risk is very low, although multiple re-infections eventually lead to blindness. The symptoms are internally scarred eyelids, followed by eyelids turning inward. Trachoma is caused by a micro-organism that spreads through eye discharges (on hands, cloth, etc.) and by "eye-seeking flies".

It is treated with antibiotics. The only known prevention method is interpersonal hygiene.

Necatoriasis is caused by "N. americanus". "N. americanus" can be divided into two areas – larvae and adult stage. The third stage larvae are guided to human skin by following thermal gradients. Typically, the larvae enter through the hands and feet following contact with contaminated soil. A papular, pruritic, itchy rash will develop around the site of entry into the human host. This is also known as "ground itch". Generally, migration through the lungs is asymptomatic but a mild cough and pharyngeal irritation may occur during larval migration in the airways. Once larvae break through the alveoli and are swallowed, they enter the gastrointestinal tract and attach to the intestinal mucosa where they mature into adult worms. The hookworms attach to the mucosal lining using their cutting plates which allows them to penetrate blood vessels and feed on the host's blood supply. Each worm consumes 30μl of blood per day. The major issue results from this intestinal blood loss which can lead to iron-deficiency anemia in moderate to heavy infections. Other common symptoms include epigastric pain and tenderness, nausea, exertional dyspnea, pain in lower extremities and in joints, sternal pain, headache, fatigue, and impotence. Death is rare in humans.

They commonly infect the skin, eyes, and viscera in humans.

- Ancylostoma brasiliensis causes cutaneous larva migrans.

- Toxocara causes visceral larva migrans.

Helminths are extremely successful parasites capable of establishing long-lasting infections within a host. During this time, helminths compete with the host organism's cells for nutrient resources and thus possess the potential to cause harm. However, the number of organisms hosted by individuals undergoing helminthic therapy is very small and any side effects are typically only encountered in the first three months of infection. In the long term, the vast majority of clinically infected individuals are asymptomatic, with no significant nutrient loss. In fact, nutrient uptake can be enhanced in some subjects who are hosting a small number of helminths. If the side effects from helminthic therapy were to become unmanageable, they can be alleviated by the use of anthelminthic medications.[1][7][8] The most common clinical symptoms which may be encountered while undergoing helminthic therapy can include:

- Fatigue

- Gastrointestinal discomfort

- Anemia

- Fever

- Abdominal pain

- Weight loss

- Anorexia

- Diarrhea

- General malaise

There is a negative association between an infection with the parasite "T. gondii" and multiple sclerosis, therefore, researchers have concluded that toxoplasmosis infection could be considered a protective factor.

Amphistomiasis in farm and wild mammals is due to infection of paramphistomes, such as the species of "Paramphistomum", "Calicophoron", "Cotylophoron", "Pseudophisthodiscus", etc. These are essentially rumen flukes, of which "Paramphistomum cervi" is the most notorious in terms of prevalence and pathogenicity. Infection occurs through ingestion of contaminated vegetables and raw meat, in which the viable infective metacercaria are deposited from snails, which are the intermediate hosts. The immature flukes are responsible for destroying the mucosal walls of the alimentary tract on their way to growing into adults. It is by this fervent tissue obliteration that the clinical symptoms are manifested. The adult flukes, on the other hand, are quite harmless, as they merely prepare for reproduction.

The zoonotic infection in human is caused by "G. discoides" and "W. watsoni" which are essentially intestinal flukes. The disease due to "G. discoides" is more specifically termed gastrodiscoidiasis. In their natural hosts such as pigs and monkeys, their infection in asymptomatic, but human infection is prevalent, by which they cause serious health problems, characterised by diarrhoea, fever, abdominal pain, colic, and an increased mucous production. In extreme situations such as in Assam, India, a number of mortality among children is attributed to this disease.

Evidence in support of the idea that helminthic infections reduce the severity of autoimmune diseases is primarily derived from animal models. Studies conducted on mice and rat models of colitis, muscular sclerosis, type 1 diabetes, and asthma have shown helminth-infected subjects to display protection from the disease. While helminths are often considered a homogenous group, considerable differences exist between species and the utilization of species in clinical research varies between human and animal trials. As such, caution must be exercised when interpreting the results from animal models.

Helminthic therapy is currently being studied as a treatment for several (non-viral) autoimmune diseases in humans including celiac disease, Crohn's disease, multiple sclerosis, ulcerative colitis, and atherosclerosis. It is currently unknown which clinical dose or species of helminth is the most effective method of treatment. Hookworms have been linked to reduced risk of developing asthma, while "Ascaris lumbricoides" (roundworm infection) was associated with an "increased" risk of asthma. Similarly, "Hymenolepis nana", "Trichoris trichiura", "Ascaris lumbricoides", "Strongyloides stercolaris", "Enterobius vermicularis", and "Trichuris suis" ova have all been found to lower the number of symptom exacerbations, reduce the number of symptom relapses, and decrease the number of new or enlarging brain lesions in patients with multiple sclerosis at doses ranging from 1,180 to 9,340 eggs per gram. However, "Ascaris lumbricoides", "Strongyloides stercolaris" and "Enterobius vermicularis" are not considered suitable for therapeutic use in humans because they do not meet the criteria for a therapeutic helminth.

"Trichuris suis" ova has been used in most cases to treat autoimmune disorders because it is thought to be non-pathogenic in humans and therefore has been rendered as safe.

The use of "Trichuris suis" ova has been granted by the USA Food and Drug Administration as an investigational medicinal product (IMP). While in the UK, the hookworm "Necator americanus" has been granted an IMP license by the Medicines and Healthcare Regulatory Authority. This hookworm is likely to be relatively safe, although it can cause temporary gastrointestinal side effects, especially following the initial inoculation and with larger doses.

The general ideal characteristics for a therapeutic helminth are as follows:

- Little or no pathogenic potential

- Does not multiply in the host

- Cannot be directly spread to close contacts

- Produces a self-limited colonization in humans

- Produces an asymptomatic colonization in humans

- Does not alter behaviour in patients with depressed immunity

- Is not affected by most commonly used medications

- Can be eradicated with an anti-helminthic drug

- Can be isolated free of other potential pathogens

- Can be isolated or produced in large numbers

- Can be made stable for transport and storage

- Easy to administer

Amphistomiasis or paramphistomiasis (alternatively spelled amphistomosis or paramphistomosis) is a parasitic disease of livestock animals, more commonly of cattle and sheep, and humans caused by immature helminthic flatworms belonging to the order Echinostomida. The term amphistomiasis is used for broader connotation implying the disease inflicted by members of Echinostomida including the family Paramphistomidae/Gastrodiscidae (to be precise, the species "Gastrodiscoides hominis"); whereas paramphistomiasis is restricted to that of the members of the family Paramphistomatidae only. "G. discoides" and "Watsonius watsoni" are responsible for the disease in humans, while most paramphistomes are responsible in livestock animals, and some wild mammals. In livestock industry the disease causes heavy economic backlashes due to poor production of milk, meat and wool.

In most of the current studies where positive correlations have been found between "T. gondii" antibody titers and certain behavioral traits or neurological disorders, "T. gondii" seropositivity tests are conducted after the onset of the examined disease or behavioral trait; that is, it is often unclear whether infection with the parasite increases the chances of having a certain trait or disorder, or if having a certain trait or disorder increases the chances of becoming infected with the parasite. Groups of individuals with certain behavioral traits or neurological disorders may share certain behavioral tendencies that increase the likelihood of exposure to and infection with "T. gondii"; as a result, it is difficult to confirm causal relationships between "T. gondii" infections and associated neurological disorders or behavioral traits. Provided there is in fact a causal link between "T. gondii" and schizophrenia, studies have yet to determine why some individuals with latent toxoplasmosis develop schizophrenia while others do not, however, some plausible explanations include differing genetic susceptibility, parasite strain differences, and differences in the route of the acquired "T. gondii" infection.

Humans contract "Blastocystis" infection by drinking water or eating food contaminated with feces from an infected human or animal. "Blastocystis" infection can be spread from animals to humans, from humans to other humans, from humans to animals, and from animals to animals. Risk factors for infection have been reported as following:

- International travel: Travel to less developed countries has been cited in development of symptomatic Blastocystis infection. A 1986 study in the United States found that all individuals symptomatically infected with "Blastocystis" reported recent travel history to less developed countries. In the same study, all hospital employees working in New York who were screened for "Blastocystis" were found to have asymptomatic infections.

- Military service: Several studies have identified high rates of infection in military personnel. An early account described infection of British troops in Egypt in 1916 who recovered following treatment with emetine. A 1990 study published in "Military Medicine" from Lackland AFB in Texas concluded symptomatic infection was more common in foreign nationals, children, and immunocompromised individuals. A 2002 study published in "Military Medicine" of army personnel in Thailand identified a 44% infection rate. Infection rates were highest in privates who had served the longest at the army base. A follow-up study found a significant correlation between infection and symptoms, and identified the most likely cause as contaminated water. A 2007 newspaper article suggested the infection rate of US military personnel returning from the Gulf War was 50%, quoting the head of Oregon State University's Biomedicine department.

- Consumption of Untreated Water (well water): Many studies have linked "Blastocystis" infection with contaminated drinking water. A 1993 study of children infected symptomatically with "Blastocystis" in Pittsburgh indicated that 75% of them had a history of drinking well water or travel in less developed countries. Two studies in Thailand linked "Blastocystis" infection in military personnel and families to drinking of unboiled and untreated water. A book published in 2006 noted that in an Oregon community, infections are more common in winter months during heavy rains. A research study published in 1980 reported bacterial contamination of well water in the same community during heavy rainfall. A 2007 study from China specifically linked infection with "Blastocystis sp. subtype 3" with drinking untreated water. Recreational contact with untreated water, for example though boating, has also been identified as a risk factor. Studies have shown that "Blastocystis" survives sewage treatment plants in both the United Kingdom and Malaysia. "Blastocystis" cysts have been shown to be resistant to chlorination as a treatment method and are among the most resistant cysts to ozone treatment.

- Contaminated Food: Contamination of leafy vegetables has been implicated as a potential source for transmission of "Blastocystis" infection, as well as other gastrointestinal protozoa. A Chinese study identified infection with "Blastocystis sp. subtype 1" as specifically associated with eating foods grown in untreated water.

- Daycare facilities: A Canadian study identified an outbreak of "Blastocystis" associated with daycare attendance. Prior studies have identified outbreaks of similar protozoal infections in daycares.

- Geography: Infection rates vary geographically, and variants which produce symptoms may be less common in industrialized countries. For example, a low incidence of "Blastocystis" infection has been reported in Japan. A study of individuals infected with "Blastocystis" in Japan found that many (43%, 23/54) carried "Blastocystis sp. subtype 2", which was found to produce no symptoms in 93% (21/23) of patients studied, in contrast to other variants which were less common but produced symptoms in 50% of Japanese individuals. Studies in urban areas of industrialized countries have found "Blastocystis" infection associated with a low incidence of symptoms. In contrast, studies in developing countries generally show "Blastocystis" to be associated with symptoms. In the United States, a higher incidence of "Blastocystis" infection has been reported in California and West Coast states.

- Prevalence over Time: A 1989 study of the prevalence of "Blastocystis" in the United States found an infection rate of 2.6% in samples submitted from all 48 states. The study was part of the CDC's MMWR Report. A more recent study, in 2006, found an infection rate of 23% in samples submitted from all 48 states. However, the more recent study was performed by a private laboratory located in the Western US, and emphasized samples from Western states, which have previously been reported to have a higher infection rate.

Research studies have suggested the following items are not risk factors for contracting "Blastocystis" infection:

- Consumption of municipal water near water plant (not a risk factor): One study showed that municipal water was free of "Blastocystis", even when drawn from a polluted source. However, samples taken far away from the treatment plant showed cysts. The researchers suggested that aging pipes may permit intrusion of contaminated water into the distribution system.

- Human-to-Human transmission among adults (not a risk factor): Some research suggests that direct human-to-human transmission is less common even in households and between married partners. One study showed different members of the same household carried different subtypes of Blastocystis.

Experimental infection in immunocompetent and immunocompromised mice has produced intestinal inflammation, altered bowel habits, lethargy and death. Chronic diarrhea has been reported in non-human higher primates.

Dientamoebiasis is a medical condition caused by infection with "Dientamoeba fragilis", a single-cell parasite that infects the lower gastrointestinal tract of humans. It is an important cause of traveler's diarrhea, chronic abdominal pain, chronic fatigue, and failure to thrive in children.