• VIRUSES are the smallest of all infectious agents, averaging about 100 nanometers (100 billionths of a meter) in length. They have so few genes and proteins of their own that in order to reproduce they need to commandeer the machinery of the cells they invade.

• BACTERIA vary widely in size and shape but tend to be at least 10 times larger than viruses, or at least 1 micrometer (1 millionth of a meter) long. They are single-cell organisms that reproduce independently.

• SINGLE-CELL ORGANISMS tend to be at least 10 times larger than bacteria, or about .01 millimeter long.

• MULTICELLULAR ORGANISMS are so large they can usually be seen with the naked eye. Tapeworms, for instance, can reach a length of 6 meters (20 feet).

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Food and water are the most common sources of parasite and invading organism transmission. Since most of us eat three times a day and drink water frequently throughout the day, our exposure to these sources is constant. Tap water has been found to be contaminated with harmful organisms. Both plant and animal foods carry parasites, and cleaning and cooking methods often do not often destroy them before ingestion. The CDC (Center for Disease Control) cites food as the catalyst behind 80 percent of the pathogenic outbreaks in the U.S. Most are linked to restaurants and delis where less than sanitary conditions exist -- from food preparation and storage to the utensils and servers' hands.

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PROTOZOA
Making up approximately 70 percent of all invading organisms, protozoa are invisible to the naked eye. They are one-celled microscopic organisms, but don't let their size fool you. Certain protoans, through their intensely rapid reproductive ability, can take over the intestinal tract of their host; and from there go on to other organs and tissues. Some feed on red blood cells. Some protozoa produce cysts - closed sacs in which they may be safely transported through food and water from one person to another. In the cyst state, protozoans are safe from destruction by human digestive juices. These one-celled 'vampires' can actually destroy the tissues of their hosts. According to experts, an estimated 7 million people across the U.S. have some form of protozoa living inside of them. Common protozoa include: Endolimax nana, Giardia lamblia, Entamoeba histolytica, Cryptosporidum parvum, Blastocystis hominis, Trichomonas vaginalis, Toxoplasma gondii, Cyclospora cayetanensis, Cryptosporidium muris, Pneumocystis carinii, Plasmodium malariae, Plasmodium ovale, Plasmodium vivax, Plasmodium falciparum, Leishmania donovani, Leishmania tropica, and Leishmania braziliensis.

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NEMATODE
While the protozoans are only single-celled, nematode creatures are multi cellular. The adult worms multiply by producing eggs called ova or larvae. The eggs usually become infectious in soil or in an intermediate host before humans are infected. It is interesting to note that unless the worm infection is heavy, many individuals do not show signs of infestation. While it may be unpleasant to consider, it is true that the human host can coexist quite comfortably with a few worms, unless they reproduce in great numbers and create organ obstruction. Experts claim that 'some type of worm is already in the intestines of over 75 percent of the world's population'. This is a frightening statement. Common nematode include: Roundworm (Ascaris lumbricoides), Hookworm (Necator Americanus, Ancylostoma duodenal), Pinworm (Enterobius vermicularis), Roundworm (Toxocara canis, Toxocara cati), Heart worm (Dirofilaria immitis), Strongyloides (Stronglyoides stercoralis), Trichinella (Trichinella spiralis), Filaria (Wuchereria bancrofti, Brugia malayi, Onchocerca volvulus, Loa loa, Mansonella streptocerca, Mansonella perstans, Mansonella ozzardi), and Anisakine larvae.

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CESTODA
Among the oldest known parasites, tapeworms are considered humanity's largest intestinal inhabitant. They each have a scolex (head) that attaches to the intestinal wall. As long as the head remains attached to the intestinal mucosa, a new worm can grow from it. Tapeworms do not contain digestive tracts but get their nourishment by absorbing partially digested substances from the host. They are whitish in color, flat, and ribbon-like, with a covering that is a transparent skin-like layer. Common cestoda include: Beef tapeworm (Taenia saginata), Pork tapeworm (Taenia solium), Fish tapeworm (Diphyllobothrium latum), and Dog tapeworm (Dipylidium caninum).

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TREMATODE
Trematode are leaf-shaped flatworms also known as flukes. They are parasitic during nearly all of their life-cycle forms. The cycle begins when larvae are released into freshwater by infected snails. The free-swimming larvae can then directly penetrate the skin of the human host or are ingested after encysting in or on various edible, vegetation, fish, or crustaceans. Common trematode include: Intestinal fluke (Fasciolopsis buski), Blood fluke (Schistosoma japonicum, Schistosoma mansoni) Schistosoma haematobium), Liver fluke (Clonorchis sinensis), Oriental lung fluke (Paragonimus westermani), and Sheep liver fluke (Fasciola hepatica).

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Six Most Common Parasites that infect the Human Body

There are different types of human parasitic worms that include pinworms, tapeworms, roundworms, hookworm, flukes, whipworms, and many others. The parasites vary in size and shape, and they cause different types of problems. They consume a significant portion of nutrients from a person leaving him to her hungry and unhealthy. Below is a list of the common parasites of the human body.

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1. Tapeworms

Infection by these intestinal parasites can be diagnosed by checking for ova and the parasite in stool samples. These worms exist in different genera that include Spirometra, Hymenolepis, Taenia, Dipylidium, Diphyllobothrium, and Echinococcus. Tapeworms do not have an intestinal tract, and therefore, the use integuments to take nutrient from the human body. Individuals who are infected can be treated by administering anthelmintic drugs such as albendazole, praziquantel, and niclosamide.

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2. Human pinworm (Enterobius vermicularis)

These intestinal parasites affect many people in the United States. The pinworms often live in a human being’s small intestines. The female parasite moves to the anus at night and lays its eggs in the person’s perianal skin folds. The larvae then travel to the lower intestine after being hatched, and they lead to retroinfection. Most people who have been infected are asymptomatic but they may trigger appendicitis in children. The pinworm is diagnosed by checking for ova and the female pinworm in perianal swabs through a microscope. The sample needs to be taken in the morning. Anthelmintic drugs such as albendazole, mebendazole, and pyrantel pamoate can be used for treatment.

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3. Ascaris

Ascariasis has been known to be the world’s most common parasitic worm infection. The Ascaris lumbricoides is a worm that grows to up to 35 centimeters long, and it is the biggest roundworm that lives in the human body. The parasite can stay in a person’s small intestines for as long two years if it is not treated. One female Ascaris lumbricoides can lay approximately 200,000 eggs in the soil, and their life cycle involves different organs that include the lungs and the liver. An infected person is asymptomatic, but in a few cases, individuals may develop malnutrition, stunted growth, hepatobiliary injury, pneumonia, and eosinophilia.

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4. Hookworms (Necator americanus and Ancylostoma duodenale)

These parasites are widespread, and they infect millions of people across the world. They have been ranked second after ascariasis in the list of the most common intestinal infections. Hookworms enter the human body by their larvae penetrating through the skin when a person comes into contact with contaminated soil. The worms travel to the lungs and then migrate to the intestines during swallowing. These parasites do not multiply in the body, and therefore, the infection can end if one is not exposed again. Ancylostoma has a lifespan of 1 to 3 years while Necator lives for 3 to 10 years. Most common symptoms include vague abdominal pain, nausea, or diarrhea, and they can be treated by using anthelmintic drugs.

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5. Intestinal Trematodes

These are also referred to as intestinal flukes, and they usually vary in size. The most common species of the flat hermaphroditic parasites include Echinostoma, Heterophyes heterophyes, and Metagonimus yokogawai. People are mainly infected after consuming an undercooked intermediate host, which may be vegetables, fish, or any other animal. The worms live on the wall of the patient’s small intestines. Most people are asymptomatic, but adult flukes lead to ulceration and inflammation. Patients can be treated by taking three doses of anthelmintic medication.

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6. Entamoeba histolytica

This parasite is a protozoan, and it leads to a disease that is known as amebiasis. It infects the human body through ingestion of contaminated food, water, or soil. The parasites of often penetrate the mucosal barrier lead to the destruction of tissues, colitis, and bloody diarrhea. The trophozoites can move to organs such as the lungs, liver, and brains through the blood hence causing the formation of abscesses. Asymptomatic amebiasis can be cured by using paromomycin or iodoquinol while tinidazole or metronidazole can be used in treating a severe infection.