Hookworm (Necator americanus)



This parasitic nematode worm begins life outside the body and is transmitted through contaminated water, or fruits and vegetables.



The hookworm larvae grow inside the human intestines where they attach to the wall of the intestine and drink the blood of the host, sometimes causing a form of anaemia called anchylostomiasis.



Symptoms: weakness, abdominal pain, nausea, diarrhoea, anaemia



(Image: Science Photo Library)

Scabies mite (Sarcoptes scabiei var. hominis)



Commonly known as the human itch mite, the parasite is transferred by physical contact.



The female mite lays her eggs on the skin of a human, causing a reaction and inflammation.



This is exacerbated when the mother begins burying the eggs under the skin causing intense itching, a condition commonly known as scabies.



Symptoms: itching, soreness, pus-filled nodules, skin irritation



(Image: Science Photo Library)

Roundworm (Ascaris lumbricoides)



These are the largest of the intestinal nematodes affecting humans, growing to 15-35 centimetres in length.



They are transferred by ingestion. The eggs hatch and quickly penetrate the intestinal wall, where they enter the bloodstream.



From there, the roundworm makes its way to the lungs, from where it is coughed up and swallowed, returning it to the gut.



Symptoms: fever, tiredness, allergic rash, vomiting, diarrhoea, nerve problems, wheezing / coughing



(Image: Science Photo Library) Advertisement

Flatworm blood fluke (Schistosoma mansoni, S. haematobium, S. japonicum)



These small flukes live in the bloodstream of infected hosts and cause schistosomiasis, also called bilharzia.



They live in water, and penetrate the skin of victims who come into contact with contaminated water.



The parasite causes inflammation (swelling) and damage to organs, particularly the liver.



The adult worms can persist in their human host for decades, and may not cause any symptoms for years.



They leave the host in faeces and spend part of their lifecycle in a snail host.



Symptoms: fever, aching, cough, diarrhoea, swollen glands, lethargy



(Image: Science Photo Library)

Tapeworm (Taenia solium)



Transmitted through infected food, a tapeworm attaches itself to its victim's intestine with hooks on its "head", or scolex.



They mature over three to four months, during which time the reproductive organs develop.



Tapeworms can survive for up to 25 years in humans.



Their eggs are excreted in faeces and can survive on vegetation, where they are then consumed by cattle or pigs, or passed on to humans.



Symptoms: nausea, vomiting, inflammation of the intestine, diarrhoea, weight loss, dizziness, fits, malnutrition



(Image: CDC)

Pinworm (Enterobius vermicularis)



Pinworms are a common human parasite, causing enterobiasis.



Adult females range from 8 to 13 millimetres in length and have a long, pin-shaped posterior, for which the worm is named.



Pinworms mate by traumatic insemination - the male stabs the female with his penis - after which the male dies.



They make their home in the host's intestines, but unlike many parasites they do not pass into the blood and cannot survive in other parts of the body for any length of time.



They lay their eggs outside the body, usually around the anus, causing itching: this assists the larvae's spread via hand contact.



Symptoms: Irritation and scratching



(Image: Public Health Image Library)

Wuchereria bancrofti



Mosquitoes carry this parasite, which they release into the bloodstream of a human host as they feed.



The larvae move to the lymph nodes, which are predominantly in the legs and genital area, and develop into an adult worm over the course of a year.



They are commonly responsible for the tropical disease filariasis, but in extreme cases can cause elephantiasis.



Symptoms: fever, chills, skin infections, painful lymph nodes, thickened skin, swelling



(Image: Sinclair Stammers/Science Photo Library)

Toxoplasma gondii



A common, crescent-shaped parasite that invades the central nervous system.



Humans become infected with this organism by eating undercooked meat or by handling infected cat litter.



Most people have been exposed to this parasite and show antibodies for it, but few individuals show symptoms.



Those with a compromised immune system are more susceptible, and fetuses can suffer serious or fatal effects from infection.



Symptoms: flu symptoms, fever, chills, fatigue, headache



(Image: Ke Hu / John Murray)

Giardia lamblia



Giardia lamblia is a flagellated protozoan parasite.



It lives and reproduces in the intestine, causing an infection of the small intestine known as giardiasis.



When it takes up residence in a human's gut, it results in inflammation and and other damage, reducing the gut's ability to absorb nutrition and causing diarrhoea.



The parasite is highly resistant to water treatment and is known to exist in drinking water.



Symptoms: diarrhoea, nausea, abdominal pain, weight loss, characteristic "rotten-egg"-tasting burps



(Image: CDC / Janice Carr)