Parasitism is a type of symbiosis ("living together"), a relationship between two organisms where one organism (the parasite) lives on or off the other (host) to the the latter's detriment. Usually the parasite, which benefits from the other organism, is physically smaller than the harmed host. Ordinarily, heavily infected hosts will be killed by their parasites.
Usually the parasite has a higher reproductive potential than its hosts, which characterizes parasites from predators. Predators have a lower reproductive potential than their prey, and are less numerous, while parasites are more numerous. 
Particularly in the field of medical parasitology, the term parasite is recognized to mean a eukaryotic pathogenic organism. Therefore, protozoan and metazoan infectious agents are classified as parasites while bacteria and viruses are not. Even though fungi are eukaryotic, they are not discussed in textbooks of medical parasitology.
Types of parasitism
Parasites that live inside the host are called endoparasites (e.g., heartworms that live in the host's heart). Those that live on or consume off the outside of its host are called ectoparasites (e.g., some mites, ticks). An epiparasite is a parasite that feeds on another parasite.
Parasitoids are parasites that consume another organism's tissue for their own nutritional benefit until the host dies from loss of needed tissues or nutrients. Biotrophic parasites cannot live in a dead host, therefore, they do not kill their host. Viruses are biotrophic, e.g., they use the host's genetic and cellular processes to survive and reproduce.
Social parasites take advantage of relationships between individuals of a social host species such as ants or termites to the other's detriment. Kleptoparasitism (parasitism by theft) involves the parasite stealing food that the host has caught or otherwise prepared. Many cuckoos use other birds as "babysitters" to feed and raise cuckoo young by adults of the host species, while adult cuckoos fend for themselves.
There are several common parasitic worms which can live in the human intestines: pinworms, tapeworms, hookworms, and roundworms. Ordinarily, intestinal worms or Helminths are multicellular and cannot multiply inside the body.
- Main Article: Hookworm
Hookworm larvae, which is hatched from eggs that are laid in soil, enter the body by burrowing through the skin, and migrate through the blood to the lungs. From the lungs they move to the throat where they are swallowed and proceed down to the intestine. The adult hookworm attaches itself the the intestinal lining with its sharp teeth and lays eggs where they will be discharged with the feces.
- Main Article: Roundworm
Roundworm eggs are passed out with feces contaminate soil, and in warm humid conditions they can develop to the stage where they become infective. Fruits and vegetables can become contaminated with the infected soil and in turn can be swallowed by humans. The eggs, once in the intestine, can hatch into larvae, where they develop and grow in the small intestine. Once adults, they move to the cecum where they lay eggs. Roundworms are more common in children and can leave the intestine to go to different parts of the body which can cause a variety of illnesses; which include pneumonia, seizures, and jaundice.
The pinworm is a type of roundworm and infection is very common in children as well as adults worldwide. Pinworms are the most common parasitic worm in the United States, and cause the infection enterobiasis in humans. Pinworms are transmitted when the eggs, which lodge under the fingernails when a person scratches, contaminate food.
- Main Article: Tapeworm
Eating insufficiently cooked meats, especially beef, pork and fish can cause tapeworm infection. Beef tapeworm, which can grow to a length of 15 to 20 feet in the intestines, is the most common tapeworm in the United States.
Protozoans are single celled parasites can multiply inside the human body, unlike parasitic worms. Protozoans can live in a number of places in the human body: gut, skin, eye, mouth, bloodstream, spleen, liver, and muscle.
Amebiasis, or amoebiasis, caused by Entamoeba histolytica can result in dysentery, liver abscess, and sometimes death. It is common in parts of Africa, South America, India, and southeast Asia. Infection is a result of ingestion of cysts in water, or on fruit and vegetables, contaminated with sewage. Several other protozoans can cause damage to the hind gut and severe diarrhea in infected individuals.
Malaria and African sleeping sickness are caused by protozoans which get into the bloodstream and infect blood cells. Only 25,000 cases of African sleeping sickness are reported per year, but 55 million individuals are estimated to be at risk. Both wild and domestic animals act as a reservoir host for the disease and the disease is transmitted by the tsetse fly (Glossina). An estimated 40% of the world's population are at risk and 10% at severe risk for malaria. Malaria is common in South America, Asia, and Africa, where it is a major cause of death in children. The disease is transmitted by mosquitoes.
Two common skin diseases, athlete's foot and ringworm, are caused by parasitic fungi. Athlete's foot is caused by a mold (although some infections are caused by a yeast) that grows on the surface of the skin and continues to grow into the living skin tissue itself, causing the infection. Ringworm (which is not caused by a worm despite its name) is a contagious skin diseases caused by a fungus that feeds on keratin.
Cryptococcosis is an infection caused by the fungi Cryptoccocus, and is especially susceptible for people with AIDS. Symptoms can include chest pain, dry cough, abdomen swelling, headache, blurred vision and confusion. Though sometimes symptoms may not be able to be detected, Cryptococcosis, especially if left untreated, can be fatal. 
Pneumocystis pneumonia is caused by a type of yeast-like fungus that usually infects people with weakened immune systems. The fungus primarily infects the lungs but has been know to spread to other organs of the body including the lymph nodes, spleen, liver, and bone marrow. The symptoms of the disease include non-productive cough, breathing difficulties, and fever. If left untreated, Pneumocystis pneumonia will cause breathing difficulties that will eventually cause death. 
- Main Article: Tick
Ticks are ectoparasites that live on the blood of humans and most animals worldwide. Ticks are prevalent vectors that can carry a wide variety of pathogens, including bacteria, protozoa, and viruses. Ticks potentially carry many serious and life-threatening diseases including: lyme disease, ehrlichiosis, babesiosis, Rocky Mountain spotted fever, tularemia, and tick-borne relapsing fever. Ticks anchor themselves firmly in place while sucking blood using their hypostome, a harpoon-like structure in their mouth area.
- Main Article: Mosquito
Mosquitoes feed on plant nectar, except for the females that feed on blood in preparation for egg-laying. Most species of mosquitoes can carry viral diseases including: yellow fever, dengue fever, epidemic polyarthritis, Rift Valley fever, Ross River fever, and West Nile virus.
- Main Article: Louse
Humans are hosts for lice which live in hair, skin and/or clothes. The commonly known head lice "glues" her eggs to hair shafts very close to the scalp. These eggs or "nits" usually take about a week to hatch into a baby louse called a nymph. The nymph must feed on blood in order to live and suck blood from human skin. It metamorphoses 3 times before it reaches the adult stage where it continues to consume blood. If the louse falls off a person, it usually dies within 1-2 days.
Fleas live on a large selection of warm-blooded vertebrates including dogs, cats, humans, chickens, rabbits, squirrels, rats and mice. Adult fleas must feed on blood before they can become capable of reproduction. Flea bites usually results as a slightly-raised swollen spot with a single puncture point at the center which causes an itching sensation to the host. Some people and animals suffer allergic reactions to flea saliva, resulting in rashes. In addition to the irritation flea bites cause, they serve as disease vectors. Fleas have been known to carry bubonic plague between rodents and humans, Murine typhus (endemic typhus) fever, and in some cases Hymenolepiasis (tapeworm).
- Parasitism by Wikipedia
- TEACHING NOTES AND RESOURCES: PARASITOLOGY
- Protozoa as Human Parasites
- William S. Pinkston, Jr. Biology for Christian Schools. South Carolina: Bob Jones University Press, 1987.
- Aspergillosis by Wikipedia
- Pneumocystis pneumonia by Wikipedia
- Cryptococcosis by Wikipedia
- Health A to Z, "Pneumocystis pneumonia"
- Tick Biology by the University of California, Davis - Department of Entomology
- Tick by Wikipedia
- Mosquito by Wikipedia
- Lice by Wikipedia
- Head louse by Wikipedia
- Flea by Wikipedia
- Brainwashed by a parasite Information and photos of insects attacked by parasitic fungi
- Parasites in Humans: "How common are parasites?" by Genesis Health Products
- Facts about Intestinal Worms by School Health & Nutrition
- TEACHING NOTES AND RESOURCES: PARASITOLOGY by Aberystwyth University Parasitology Group
- A-Z Index of Parasitic Diseases by the Center for Disease Control.