Mammals can get parasites from contaminated food or water, bug bites, or sexual contact. Ingestion of contaminated water can produce Giardia infections.

Parasites normally enter the body through the skin or mouth. Close contact with pets can lead to parasite infestation as dogs and cats are host to many parasites.

Other risks that can lead people to acquire parasites are walking barefeet, inadequate disposal of feces, lack of hygiene, close contact with someone carrying specific parasites, and eating undercooked foods, unwashed fruits and vegetables or foods from contaminated regions.

Parasites can also be transferred to their host by the bite of an insect vector, i.e. mosquito, bed bug, fleas.

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.

Coccidiosis is a significant disease for chickens, especially affecting the young chicks. It can be fatal or leave the bird with compromised digestion. There are chick feed mixes that contain a coccidiostat to manage exposure levels and control disease. In an outbreak, coccidiocidal medications are given. Examples are toltrazuril (Baycox) or amprolium. After multiple infections, surviving chickens become resistant to the coccidia.

The World Health Organization estimates that globally more than 1.5 billion people (24% of the total population) have a soil-transmitted helminth infection. Over 270 million preschool-age children and over 600 million school-age children live in areas where these parasites are intensively transmitted, and are in need of treatment and preventive interventions. Latest estimates indicate that more than 880 million children are in need of treatment from STH infections.

By type of parasitic worm the breakdown is:

- approximately 807-1,121 million with ascaris

- approximately 576-740 million with hookworm

- approximately 604-795 million with whipworm

The most common medications used to treat coccidian infections are in the sulfonamide antibiotic family.

Depending on the pathogen and the condition of the animal, untreated coccidiosis may clear of its own accord, or become severe and damaging, and sometimes cause death.

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.

Good hygiene is necessary to avoid reinfection. The Rockefeller Foundation's hookworm campaign in Mexico in the 1920s was extremely effective at eliminating hookworm from humans with the use of anthelmintics. However, preventative measures were not adequately introduced to the people that were treated. Therefore, the rate of reinfection was extremely high and the project evaluated through any sort of scientific method was a marked failure. More education was needed to inform the people of the importance of wearing shoes, using latrines (better access to sanitation), and good hygiene.

Intestinal parasite prevention methods are not isolated to specific geographical areas; however, many of the research-based interventions have primarily taken place in underdeveloped countries and regions, where sanitation is a large concern for spreading disease.Current best practice behaviors that prevent intestinal parasites include: using proper hand washing practices, using correctly-built latrines with ample ventilation, having a piped water source, and wearing shoes. Currently, in some parts of Ethiopia where disease prevalence is high, up to 80% of people in a population lack access to washing facilities. While is this high, 93% did have access to a latrine, but only 29.2% of those latrines had proper construction to decrease parasitic infections.Behavioral interventions have focused on promoting washing, sometimes with soap, in context of education at schools and child care facilities. In recent studies, the best interventions follow a multidisciplinary approach by:

- Increasing environmental sanitation to promote hand washing and shoe wearing habits

- Educating children at young ages at school and at home

Specific evidence-based interventions that may lower disease prevalence include:

- Interventions at schools, focusing on the construction of pit latrines (ventilated and improved), providing clean drinking water and educating the students about hygiene

- The SAFE (surgery, antibiotics, facial cleanliness, environmental sanitation) strategy to address trachoma, primarily the facial cleanliness and the environmental sanitation components

- Hand-washing with soap at critical times and nail clipping to decrease reinfection rates, although further research is needed to develop and implement similar interventions at scale

- Programs combining anthelmintic drug administration with interventions to increase environmental sanitation (such as decreasing fecal contamination)

Metagonimiasis infections are endemic or potentially endemic in 19 countries including Japan, Korea, China, Taiwan, the Balkans, Spain, Indonesia, the Philippines and Russia. Human infections outside endemic areas may result from ingesting pickled fish or sushi made from fish imported from endemic areas.

The prevalence of intestinal parasites is the highest among children that are living in the poorest communities in developing nations. The most common causes of intestinal parasites are through consumption of contaminated water, infected soil, inadequate sanitation and hygiene, and improper hygiene. Specifically, lack of access to facilities for safe disposal of human waste can result in intestinal parasites and disease. Poor hygiene habits or lacking available hygiene resources, such as hand washing facilities, also negatively impact rates of disease. Parasitic contamination can also occur from eating raw vegetables and fruits, soil-eating behavior, and lack of available safe water.

Parasites can get into the intestine by going through the mouth from uncooked or unwashed food, contaminated water or hands, or by skin contact with larva infected soil; they can also be transferred by the sexual act of anilingus in some cases.

When the organisms are swallowed, they move into the intestine, where they can reproduce and cause symptoms. Children are particularly susceptible if they are not thoroughly cleaned after coming into contact with infected soil that is present in environments that they may frequently visit such as sandboxes and school playgrounds. People in developing countries are also at particular risk due to drinking water from sources that may be contaminated with parasites that colonize the gastrointestinal tract.

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.

Pinworm infection occurs worldwide, and is the most common helminth (i.e., parasitic worm) infection in the United States and Western Europe. In the United States, a study by the Center of Disease Control reported an overall incidence rate of 11.4% among people of all ages. Pinworms are particularly common in children, with prevalence rates in this age group having been reported as high as 61% in India, 50% in England, 39% in Thailand, 37% in Sweden, and 29% in Denmark. Finger sucking has been shown to increase both incidence and relapse rates, and nail biting has been similarly associated. Because it spreads from host to host through contamination, enterobiasis is common among people living in close contact, and tends to occur in all people within a household. The prevalence of pinworms is not associated with gender, nor with any particular social class, race, or culture. Pinworms are an exception to the tenet that intestinal parasites are uncommon in affluent communities.

Several public health prevention strategies could help lower the rates of metagonimiasis. One is to control the intermediate host (snails). This can be done through use of molluscidals. Another is to use education to ensure all people, especially in areas were the disease regularly occurs, fully cook all fish. This could potentially be problematic and not as effective as hoped as many of the people affected by metagonimiasis eat raw or pickled fish as part of a traditional, long-seated dietary practice. Additionally, implementing more sanitary water conditions would reduce the continual reintroduction of eggs to water sources, thus restarting the lifecycle. Complete control of metagonimiasis presents several potential problems because it does have several reservoir hosts, thus eradication is unlikely.

Parasitic infections can usually be treated with antiparasitic drugs.

Albendazole and mebendazole have been the treatments administered to entire populations to control hookworm infection. However, it is a costly option and both children and adults become reinfected within a few months after deparasitation occurs raising concerns because the treatment has to repeatedly be administered and drug resistance may occur.

Another medication administered to kill worm infections has been pyrantel pamoate. For some parasitic diseases, there is no treatment and, in the case of serious symptoms, medication intended to kill the parasite is administered, whereas, in other cases, symptom relief options are used. Recent papers have also proposed the use of viruses to treat infections caused by protozoa.

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

Humans are accidental hosts of "Toxocara", yet toxocariasis is seen throughout the world. Most cases of toxocariasis are seen in people under the age of twenty. Seroprevalence is higher in developing countries, but can be considerable in first world countries, as well. In Bali, St. Lucia, Nepal and other countries, seroprevalence is over fifty percent. Previous to 2007, the U.S. seroprevalence was thought to be around 5% in children. However, Won et al. discovered that U.S. seroprevalence is actually 14% for the population at large. In many countries, toxocariasis is considered very rare. Approximately 10,000 clinical cases are seen a year in the U.S., with ten percent being OLM. Permanent vision loss occurs in 700 of these cases.

Young children are at the greatest risk of infection because they play outside and tend to place contaminated objects and dirt in their mouths. Dog ownership is another known risk factor for transmission. There is also a significant correlation between high "Toxocara" antibody titers and epilepsy in children.

Parasitic loads as high as 300 larvae in a single gram of liver have been noted in humans. The “excretory-secretory antigens of larvae… released from their outer epicuticle coat [and]… readily sloughed off when bound by specific antibodies” incite the host’s immune response. The tipping point between development of VLM and OLM is believed to be between 100 and 200 larvae. The lighter infection in OLM is believed to stimulate a lower immune response and allow for migration of a larva into the eye. Larvae are thought to enter the eye through the optic nerve, central retinal artery, short posterior ciliary arteries, soft tissues, or cerebrospinal fluid. Ocular granulomas that form around a larva typically are peripheral in the retina or optic disc.

Visceral larva migrans seems to affect children aged 1–4 more often while ocular larva migrans more frequently affects children aged 7–8. Between 4.6% and 23% of U.S. children have been infected with the dog roundworm egg. This number is much higher in other parts of the world, such as Colombia, where up to 81% of children have been infected.

They are treated with antiprotozoal agents. Recent papers have also proposed the use of viruses to treat infections caused by protozoa.

Actively involving veterinarians and pet owners is important for controlling the transmission of "Toxocara" from pets to humans. A group very actively involved in promoting a reduction of infections in dogs in the United States is the Companion Animal Parasite Council -- CAPC. Since pregnant or lactating dogs and cats and their offspring have the highest, active parasitic load, these animals should be placed on a deworming program. Pet feces should be picked up and disposed of or buried, as they may contain "Toxocara" eggs. Practicing this measure in public areas, such as parks and beaches, is especially essential for decreasing transmission. Up to 20% of soil samples of U.S. playgrounds have found roundworm eggs. Also, sandboxes should be covered when not in use to prevent cats from using them as litter boxes. Hand washing before eating and after playing with pets, as well as after handling dirt will reduce the chances of ingesting "Toxocara" eggs. Washing all fruits and vegetables, keeping pets out of gardens and thoroughly cooking meats can also prevent transmission. Finally, teaching children not to place nonfood items, especially dirt, in their mouths will drastically reduce the chances of infection.

Toxocariasis has been named one of the neglected diseases of U.S. poverty, because of its prevalence in Appalachia, the southern U.S., inner city settings, and minority populations. Unfortunately, there is currently no vaccine available or under development. However, the mitochondrial genomes of both "T. cati" and "T. canis" have recently been sequenced, which could lead to breakthroughs in treatment and prevention.

By one author's count, 257 human cases of "Thelazia callipaeda" had been reported worldwide by the year 2000, though thelaziasis is still considered to be a rare disease.

Various livestock and wildlife surveys suggest that thelaziasis is quite common among animals.

- A slaughterhouse survey in Canada found that about one-third (32%) of cattle over an 8-month period were infested with eyeworms.

- A survey of horses in Kentucky revealed a 42% rate of infestation with "Thelazia lacrymalis".

- In Wyoming and Utah, a survey of hunter-harvested mule deer found 15% to be infested by "Thelazia californiensis".

- A survey of various sites in Italy found 23-60% of dogs, 5% of foxes and 4 out of 4 cats to be infested with "Thelazia callipaeda".

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.

Sparganosis is endemic or potentially endemic in 48 countries, and although rare, cases have been described in Asia, Africa, Australia, South America, and the United States. The majority of cases occur in Southeast Asia and Eastern Africa. Ocular sparganosis is especially prevalent in China and Vietnam. The highest numbers of cases occur in Korea and Japan. As of 2003, only seven cases of sparganosis had ever been described in Europe.

Because of Eustrongylides species’ complex life cycle with various host species, preventing infection and controlling outbreaks is difficult. Outbreaks of this disease are closely linked to agricultural runoff and urban development Eutrophication of water bodies supports high population levels of oligochaete worms, which causes increased numbers of infected fish that eat the worms, and then the birds who eat the fish.

One way to prevent Eustrongylidosis is to control oligochaete populations. Outbreaks of this parasite are closely linked to high numbers of oligochaete worms in the area’s waterways. This is because the worms are essential for Eustrongylides species to reproduce. Oligochaete populations can be controlled by monitoring nutrient levels in the water, because high nutrient levels support oligochaete populations. They can also be controlled by decreasing the level of oxygen in the water. Encouraging responsible farming practices in order to reduce chemical run-off will help prevent this disease from occurring.

Managers need to be diligent in catching the symptoms of the parasite before it can become an outbreak. Once an outbreak of Eustrongylidosis has occurred, there is little that ecosystem managers can do to stop the spread in oligochaetes, fish and birds. Traditional anthelminthics (dewormers) are not effective in fish because they kill parasites that live inside the gastrointestinal tract, whereas Eustrongylides species live outside the stomach in the body cavity. The parasites can only be removed from fish surgically, which is not feasible. In order to completely stop the Eustrongylides life cycle in fish, all fish in an affected area must be culled.

Surgical removal of the parasite from wading birds is a viable option, but this would also not be feasible for a large number of birds, and it would not stop the cycle of infection.

Pinworm infection cannot be totally prevented under most circumstances. This is due to the prevalence of the parasite and the ease of transmission through soiled night clothes, airborne eggs, contaminated furniture, toys and other objects. Infection may occur in the highest strata of society, where hygiene and nutritional status are typically high. The stigma associated with pinworm infection is hence considered a possible over-emphasis. Counselling is sometimes needed for upset parents that have discovered their children are infected, as they may not realize how prevalent the infection is.

Preventative action revolves around personal hygiene and the cleanliness of the living quarters. The "rate" of reinfection can be reduced through hygienic measures, and this is recommended especially in recurring cases.

The main measures are keeping fingernails short, and washing and scrubbing hands and fingers carefully, especially after defecation and before meals. Under ideal conditions, bed covers, sleeping garments, and hand towels should be changed daily. Simple laundering of clothes and linen disinfects them. Children should wear gloves while asleep, and the bedroom floor should be kept clean. Food should be covered to limit contamination with dust-borne parasite eggs. Household detergents have little effect on the viability of pinworm eggs, and cleaning the bathroom with a damp cloth moistened with an antibacterial agent or bleach will merely spread the still-viable eggs. Similarly, shaking clothes and bed linen will detach and spread the eggs.

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