Signs and symptoms depend on the type of infection. Intestinal parasites produce a variety of symptoms in those affected, most of which manifest themselves in gastrointestinal complications and general weakness. Gastrointestinal complications include diarrhea, nausea, dysentery, and abdominal pain. These symptoms negatively impact nutritional status, including decreased absorption of micronutrients, loss of appetite, weight loss, and intestinal blood loss that can often result in anemia. It may also cause physical and mental disabilities, delayed growth in children, and skin irritation around the anus and vulva.

Frequently asymptomatic. Gastrointestinal system symptoms include abdominal pain and diarrhea. Pulmonary symptoms (including Löffler's syndrome) can occur during pulmonary migration of the filariform larvae. Dermatologic manifestations include urticarial rashes in the buttocks and waist areas as well as larva currens. Eosinophilia is generally present.

Strongyloidiasis can become chronic and then become completely asymptomatic.

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.

Symptoms vary from none to severe diarrhea with poor absorption of nutrients. It can result in weakness, loss of appetite, stomach cramps, vomiting, bloating, excessive gas, and burping. Symptoms typically develop 9–15 days after exposure, but may occur as early as one day.

Symptoms are caused by "Giardia" organisms infecting the cells of the duodenum and jejunum of the small intestine and blocking nutrient absorption. Most people are asymptomatic; only about a third of those infected exhibit symptoms. If the infection is not treated, these symptoms may last for six weeks or more.

Symptomatic infections are well recognized as causing lactose intolerance, which, while usually temporary, may become permanent. Although hydrogen breath tests indicate poorer rates of carbohydrate absorption in those asymptomatically infected, such tests are not diagnostic of infection. It has been suggested that these observations are explained by symptomatic giardia infection allowing for the overgrowth of other bacteria.

Some studies have shown giardiasis should be considered as a cause of vitamin B deficiency as a result of the problems caused within the intestinal absorption system.

An intestinal parasite infection is a condition in which a parasite infects the gastro-intestinal tract of humans and other animals. Such parasites can live anywhere in the body, but most prefer the intestinal wall.

Routes of exposure and infection include ingestion of undercooked meat, drinking infected water, fecal-oral transmission and skin absorption.

Some types of helminths and protozoa are classified as intestinal parasites that cause infection—those that reside in the intestines. These infections can damage or sicken host (humans or other animals). If the intestinal parasite infection is caused by helminths, the infection is called helminthiasis.

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.

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.

Giardiasis, popularly known as beaver fever, is a parasitic disease caused by "Giardia lamblia". About 10% of those infected have no symptoms. When symptoms occur they may include diarrhea, abdominal pain, and weight loss. Vomiting, blood in the stool, and fever are less common. Symptoms usually begin 1 to 3 weeks after exposure and without treatment may last up to six weeks.

Giardia usually spreads when "Giardia lamblia" cysts within feces contaminate food or water which is then eaten or drunk. It may also spread between people and from other animals. Risk factors include travel in the developing world, changing diapers, eating food without cooking it, and owning a dog. Cysts may survive for nearly three months in cold water. Diagnosis is via stool tests.

Prevention is typically by improved hygiene. Those without symptoms do not usually need treatment. When symptoms are present treatment is typically with either tinidazole or metronidazole. People may become temporarily lactose intolerant after an infection and therefore it is often recommended milk be avoided for a few weeks. Resistance to treatment may occur.

Giardia is one of the most common parasitic human diseases globally. In 2013, there were about 280 million people worldwide with symptomatic giardiasis. Rates are as high as 7% in the developed world and 30% in the developing world. The World Health Organization classified it as a neglected disease.

Infection causes acute, non-bloody diarrhea with crampy abdominal pain, which can last for weeks and result in malabsorption and weight loss. In immunodepressed patients, and in infants and children, the diarrhea can be severe. Eosinophilia may be present (differently from other protozoan infections).

human intestinal spirochetosis, also intestinal spirochetes, colonic spirochetosis and colonic spirochetes, is an infection of the colonic-type mucosa with spirochete microorganisms.

Isosporiasis, also known as cystoisosporiasis, is a human intestinal disease caused by the parasite "Isospora belli" (now known as "Cystoisospora belli"). It is found worldwide, especially in tropical and subtropical areas. Infection often occurs in immuno-compromised individuals, notably AIDS patients, and outbreaks have been reported in institutionalized groups in the United States. The first documented case was in 1915. It is usually spread indirectly, normally through contaminated food or water (CDC.gov).

No clear association exists with complaints. However, potential associations exist with include watery diarrhea and abdominal pain, which may be seen with blood; these findings are not specific, i.e. may be seen due to number of other causes.

Helminths (), also commonly known as parasitic worms, are large multicellular organisms, which can generally be seen with the naked eye when they are mature. They are often referred to as intestinal worms even though not all helminths reside in the intestines. For example, schistosomes are not intestinal worms, but rather reside in blood vessels. The word helminth comes from Greek "hélmins", a kind of worm.

There is no consensus on the taxonomy of helminths. It is simply a commonly used term to describe certain worms with some similarities. These are flatworms (platyhelminthes), namely cestodes (tapeworms) and trematodes (flukes), and roundworms or nemathelminths (nematodes) – both of these are parasitic worm types – and the annelida, which are not parasitic or at the most ectoparasites like the leeches.

Helminths are worm-like organisms living in and feeding on living hosts. They receive nourishment and protection while disrupting their hosts' nutrient absorption. This can cause weakness and disease of the host. Those helminths that live inside the digestive tract are called intestinal parasites. They can live inside humans and other animals. In their adult form, helminths cannot multiply in humans. Helminths are able to survive in their mammalian hosts for many years due to their ability to manipulate the immune response by secreting immunomodulatory products. All helminths produce eggs (also called ova) for reproduction. These eggs have a strong shell that protects them against a range of environmental conditions. The eggs can therefore survive in the environment, outside their hosts, for many months or years.

Many, but not all, of the worms referred to as helminths belong to the group of intestinal parasites. An infection by a helminth is known as helminthiasis, helminth infection or intestinal worm infection. There is a naming convention which applies to all helminths: the ending "-asis" (or in veterinary science: "-osis") is added at the end of the name of the worm to denote the infection with that particular worm. For example, "Ascaris" is the name of a type of helminth, and ascariasis is the name of the infectious disease caused by that helminth.

Tropical sprue is a malabsorption disease commonly found in tropical regions, marked with abnormal flattening of the villi and inflammation of the lining of the small intestine.

It differs significantly from coeliac sprue. It appears to be a more severe form of environmental enteropathy.

The illness usually starts with an attack of acute diarrhoea, fever and malaise following which, after a variable period, the patient settles into the chronic phase of diarrhoea, steatorrhoea, weight loss, anorexia, malaise, and nutritional deficiencies.

The symptoms of tropical sprue are:

- Diarrhoea

- Steatorrhoea or fatty stool (often foul-smelling and whitish in colour)

- Indigestion

- Cramps

- Weight loss and malnutrition

- Fatigue

Left untreated, nutrient and vitamin deficiencies may develop in patients with tropical sprue. These deficiencies may have these symptoms:

- Vitamin A deficiency: hyperkeratosis or skin scales

- Vitamin B and folic acid deficiencies: anaemia

- Vitamin D and calcium deficiencies: spasm, bone pain, numbness, and tingling sensation

- Vitamin K deficiency: bruises

Symptoms of ileus include, but are not limited to:

- moderate, diffuse abdominal discomfort

- constipation

- abdominal distension

- nausea/vomiting, especially after meals

- vomiting of bilious fluid

- lack of bowel movement and/or flatulence

- excessive belching

Neonatal bowel obstruction (NBO) or neonatal intestinal obstruction is the most common surgical emergency in the neonatal period. It may occur due to a variety of conditions and has an excellent outcome based on timely diagnosis and appropriate intervention.

There is no real consensus on the taxonomy (or groupings) of the helminths, particularly with the nematodes. The term "helminth" contains a number of phyla, many of which are completely unrelated. However, for practical considerations the term is still used nowadays to describe four groups with superficial similarities, the phyla Annelida, Platyhelminths, Nematoda and Acanthocephala.

There is in fact no helminth classification; it is an "artificial" term.

The most important helminths in the sanitation field are the human parasites, which is why most people relate the term helminth to them, where they are classified as nemathelminthes (nematodes) and platyhelminthes, depending on whether they possess a round or flat-shaped body respectively. The latter are further divided into cestodes and trematodes depending on whether or not they have a segmented body.

Ringworm (dermatophytosis) is actually caused by various fungi and not by a parasitic worm.

Clinical features of intestinal pseudo-obstruction can include abdominal pain, nausea, severe distension, vomiting, dysphagia, diarrhea and constipation, depending upon the part of the gastrointestinal tract involved. In addition, in the moments in which abdominal colic occurs, an abdominal x-ray shows intestinal air fluid level. All of these features are also similar in true mechanical obstruction of the bowel.

Signs and symptoms of enteritis are highly variable and vary based on the specific cause and other factors such as individual variance and stage of disease.

Symptoms may include abdominal pain, cramping, diarrhoea, dehydration, fever, nausea, vomiting and weight loss.

The diagnosis is suspected based on polyhydramnios in uteru, bilious vomiting, failure to pass meconium in the first day of life, and abdominal distension. The presentations of NBO may vary. It may be subtle and easily overlooked on physical examination or can involve massive abdominal distension, respiratory distress and cardiovascular collapse. Unlike older children, neonates with unrecognized intestinal obstruction deteriorate rapidly.

Intestinal pseudo-obstruction is a clinical syndrome caused by severe impairment in the ability of the intestines to push food through. It is characterized by the signs and symptoms of intestinal obstruction without any lesion in the intestinal lumen. Clinical features can include abdominal pain, nausea, severe distension, vomiting, dysphagia, diarrhea and constipation, depending upon the part of the gastrointestinal tract involved. The condition can begin at any age and it can be a primary condition (idiopathic or inherited) or caused by another disease (secondary).

It can be chronic or acute.

Decreased propulsive ability may be broadly classified as caused either by bowel obstruction or intestinal atony or paralysis. However, instances with symptoms and signs of a bowel obstruction occur, but with the absence of a mechanical obstruction, mainly in acute colonic pseudo-obstruction, Ogilvie's syndrome.

Regardless of cause, volvulus causes symptoms by two mechanisms:

- Bowel obstruction manifested as abdominal distension and bilious vomiting.

- Ischemia (loss of blood flow) to the affected portion of intestine.

Depending on the location of the volvulus, symptoms may vary. For example, in patients with a cecal volvulus, the predominant symptoms may be those of a small bowel obstruction (nausea, vomiting and lack of stool or flatus), because the obstructing point is close to the ileocecal valve and small intestine. In patients with a sigmoid volvulus, although abdominal pain may be present, symptoms of constipation may be more prominent.

Volvulus causes severe pain and progressive injury to the intestinal wall, with accumulation of gas and fluid in the portion of the bowel obstructed. Ultimately, this can result in necrosis of the affected intestinal wall, acidosis, and death. This is known as a closed loop obstruction because there exists an isolated ("closed") loop of bowel. Acute volvulus often requires immediate surgical intervention to untwist the affected segment of bowel and possibly resect any unsalvageable portion.

Volvulus occurs most frequently in middle-aged and elderly men. Volvulus can also arise as a rare complication in persons with redundant colon, a normal anatomic variation resulting in extra colonic loops.

Sigmoid volvulus is the most-common form of volvulus of the gastrointestinal tract. and is responsible for 8% of all intestinal obstructions. Sigmoid volvulus is particularly common in elderly persons and constipated patients. Patients experience abdominal pain, distension, and absolute constipation.

Cecal volvulus is slightly less common than sigmoid volvulus and is associated with symptoms of abdominal pain and small bowel obstruction.

Volvulus can also occur in patients with Duchenne muscular dystrophy due to the smooth muscle dysfunction.

Fecal vomiting, also called feculent vomiting and stercoraceous vomiting is a kind of vomiting wherein the material vomited is of fecal origin. It is a common symptom of gastrojejunocolic fistula and intestinal obstruction in the ileum. Fecal vomiting is often accompanied by an odor of feces on the breath and other gastrointestinal symptoms, including abdominal pain, abdominal distension, dehydration, and diarrhea. In severe cases of bowel obstruction or constipation (such as those related to Clozapine-treatment) fecal vomiting has been identified as a cause of death.

Fecal vomiting occurs when the bowel is obstructed for some reason, and intestinal contents cannot move normally. Peristaltic waves occur in an attempt to decompress the intestine, and the strong contractions of the intestinal muscles push the contents backwards through the pyloric sphincter into the stomach, where they are then vomited.

Fecal vomiting can also occur in cats.

Fecal vomiting does not include vomiting of the proximal small intestine contents, which commonly occurs during vomiting.