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.

Diagnosis usually involves finding the eggs and/or adults of "C. philippinensis" in stool samples.

Symptoms in infested humans include watery diarrhea, abdominal pain, edema, weight loss, borborygmus (stomach growling), and depressed levels of potassium and albumin in the blood. In humans, the parasites damage the cells of the intestinal wall. This damage interferes with the absorption of nutrients and the maintenance of a proper electrolyte balance. Untreated "C. philippinensis" infestations are often fatal.

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.

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).

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.

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).

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.

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.

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.

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.

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

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.

Capillariasis is a disease caused by nematodes in the "Capillaria" genus. The two principal forms of the disease are:

- Intestinal capillariasis, caused by "Capillaria philippinensis"

- Hepatic capillariasis, caused by "Capillaria hepatica"

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

Dysentery is a type of gastroenteritis that results in diarrhea with blood. Other symptoms may include fever, abdominal pain, and a feeling of incomplete defecation.

It is caused by several types of infections such as bacteria, viruses, parasitic worms, or protozoa. The mechanism is an inflammatory disorder of the intestine, especially of the colon.

The most common form of dysentery is bacillary dysentery, which is typically a mild illness, causing symptoms normally consisting of mild stomach pains and frequent passage of stool or diarrhea. Symptoms normally present themselves after one to three days, and are usually no longer present after a week. The frequency of urges to defecate, the large volume of liquid feces passed, and the presence of mucus, pus, and blood depends on the pathogen causing the disease. Temporary lactose intolerance can occur, as well. In some caustic occasions severe abdominal pain, fever, shock, and delirium can all be symptoms.

In extreme cases, dysentery patients may pass more than one litre of fluid per hour. More often, individuals will complain of nausea, abdominal pain, and frequent watery and usually foul-smelling diarrhea, accompanied by mucus, blood, rectal pain, and fever. Vomiting, rapid weight-loss, and generalized muscle aches sometimes also accompany dysentery. On rare occasions, the amoebic parasite will invade the body through the bloodstream and spread beyond the intestines. In such cases, it may more seriously infect other organs such as the brain, lungs, and most commonly the liver.

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.

Symptoms

- Pain right hypochondrium referred to right shoulder

- Pyrexia (100.4 F)

- Profuse sweating and rigors

- Loss of weight

- Earthy complexion

Signs

- Pallor

- Tenderness and rigidity in right hypochondrium

- Palpable liver

- tenderness

- Basal lung signs

Most of the symptoms of BLS are non specific but nevertheless warrant the utmost attention. These include:

- Loss of appetite

- Nausea

- Flatulence

- Diarrhea

- Fullness after a meal

- Fatty stools (steatorrhea)

- Unintentional weight loss

- Generalised weakness

As a result of the concomitant vitamin and mineral deficiencies that occur as a result of the malabsorption associated with BLS patients with advanced cases should be investigated for:

- Vitamin B12 deficiency

- Folate deficiency

- Iron deficiency

- Vitamin E deficiency

Blind loop syndrome (BLS), commonly referred to in the literature as small intestinal bacterial overgrowth (SIBO) or bacterial overgrowth syndrome (BOS), is a state that occurs when the normal bacterial flora of the small intestine proliferates to numbers that cause significant derangement to the normal physiological processes of digestion and absorption. In some cases of blind loop syndrome, overgrowth of pathogenic non-commensal bacteria has also been noted. It has long been understood that from birth, and throughout life, large amounts of bacteria reside symbiotically within animal gastrointestinal tracts such as the human gastrointestinal tract. The understanding of this gut flora has even led to novel treatments for bowel irregularity that utilize so called "probiotics" or good bacteria that aid in normal digestion.

The problem of BLS arises when the bacterial colonies residing in the upper gastrointestinal tract begin to grow out of control or are altered in their makeup thereby creating a burden on the normal physiological processes occurring in the small intestine. This results in problems inclusive of but not restricted to vitamin B12 deficiency, fat malabsorption and steatorrhea, fat-soluble vitamin deficiencies and intestinal wall injury.

A amoebic liver abscess is a type of liver abscess caused by amebiasis. It is the involvement of liver tissue by trophozoites of the organism "Entamoeba histolytica" and of is abscess due to necrosis.

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 condition is typically seen in premature infants, and the timing of its onset is generally inversely proportional to the gestational age of the baby at birth (i.e. the earlier a baby is born, the later signs of NEC are typically seen). Initial symptoms include feeding intolerance, increased gastric residuals, abdominal distension and bloody stools. Symptoms may progress rapidly to abdominal discoloration with intestinal perforation and peritonitis and systemic hypotension requiring intensive medical support.