WATCH: The Most Terrible Parasites of All The Time(VIDEO)

What is a Parasite? What do Parasites do?

A parasite is an organism that lives in another organism, called the host, and often harms it. It is dependent on its host for survival – it has to be in the host to live, grow and multiply. A parasite cannot live independently.

Although a parasite rarely kills the host, in some cases it can happen. To a certain extent, if a parasite does kill its host it has failed as this means it will need to find a new home. The parasite benefits at the expense of the host – the parasite uses the host to gain strength, and the host loses some strength as a result.

Fast facts on parasites

Here are some key points about parasites. More detail and supporting information is in the main article.


  • Parasites live within other organisms and thrive to the detriment of their host
  •  Malaria is caused by a parasite
  • There are at least 1,000 species of parasite capable of living in or on humans
  • Parasites that live on the surface of their hosts are known as ectoparasites
  • Sleeping sickness is caused by a parasite transmitted by the tsetse fly
  • Some parasitic worms can grow to over 30 m in length
  • Epiparasites are parasites that live on other parasites
  • Babesiosis is a parasitic disease that affects the red blood cells
  •  Trichomonas vaginalis is a sexually transmitted parasite
  • Humans can become infected with clonorchiasi after ingesting contaminated ants.

What is a parasite?

The word “parasite” comes from the Greek “parasitos”, with para meaning “alongside”, and sitos meaning “food” – therefore meaning “eating at the side of, as one would when seated at the same table”.

It was in the 18th century that the word parasite entered the English language as a biologic term; before that it referred to humans, such as a relative or friend who lived at the expense of another person.

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In general, a parasite is an organism that live within another, using the resources of the host to fuel its life cycle.

Parasites are an incredibly varied group of organisms. Around 70% of parasites are microscopic in size, such as the malarial parasite; however, some worm parasites can reach over 30 m in length.

There are three main types of parasitic diseases:

  • Protozoa: a single-celled organism. Plasmodium, which causes malaria, is an example. A protozoa can only multiply (divide) within the host
  • Helminths: worm parasites. Schistosomiasis is caused by a helminth. Other examples include roundworm, pinworm, trichina spiralis, tapeworm, and fluke
  • Ectoparasites: these live on, rather than in their hosts, these include lice and fleas.

Parasites, unlike predators, are usually much smaller than their host and they reproduce at a faster rate.

Symptoms of parasitic diseases

Because there are so many species of parasite there is a wide array of potential symptoms. Sometimes the symptoms may appear similar to hormone deficiency, pneumonia or food poisoning. These are some of the potential symptoms that might occur:

  • Itchy anus or vagina
  • Weight loss and/or increased appetite
  • Abdominal pain
  • Diarrhea, and vomiting
  • Sleeping problems
  • Anemia
  • Aches and pains
  • General malaise
  • Allergies
  • Anxiety.

Types of Parasites

There are over 1,000 known parasite species that can infect humans. These are some examples:

Endoparasite: live inside the host (as opposed to ectoparasites), examples include heartworm, tapeworm, and flatworms. Those that inhabit the spaces within the host’s body are called intercellular parasites, while an intracellular parasite live within the host’s cells.

Intracellular parasites include bacteria and viruses; they rely on a third organism, known as the vector (carrier). The vector transmits the endoparasite to the host. The mosquito is a vector for many parasites including the protozoan of the genus Plasmodium that causes malaria.

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Epiparasite: these feed on other parasites, a relationship known as hyperparasitism. A flea which lives on a dog may have a protozoan in its digestive tract, the protozoan is the hyperparasite.

Parasitoid: the larval development takes place in or on another organism, the host usually dies. In this case there are characteristics of predation, because the host dies.

WATCH: The Most Terrible Parasites of All The Time

A parasite is an organism that lives on or in a host organism and gets its food from or at the expense of its host. There are three main classes of parasites that can cause disease in humans: protozoa, helminths, and ectoparasites.


Protozoa are microscopic, one-celled organisms that can be free-living or parasitic in nature. They are able to multiply in humans, which contributes to their survival and also permits serious infections to develop from just a single organism. Transmission of protozoa that live in a human’s intestine to another human typically occurs through a fecal-oral route (for example, contaminated food or water or person-to-person contact). Protozoa that live in the blood or tissue of humans are transmitted to other humans by an arthropod vector (for example, through the bite of a mosquito or sand fly).
The protozoa that are infectious to humans can be classified into four groups based on their mode of movement:

  • Sarcodina – the ameba, e.g., Entamoeba
  • Mastigophora – the flagellates, e.g., Giardia, Leishmania
  • Ciliophora – the ciliates, e.g., Balantidium
  • Sporozoa – organisms whose adult stage is not motile e.g., Plasmodium, Cryptosporidium


Helminths are large, multicellular organisms that are generally visible to the naked eye in their adult stages. Like protozoa, helminths can be either free-living or parasitic in nature. In their adult form, helminths cannot multiply in humans. There are three main groups of helminths (derived from the Greek word for worms) that are human parasites:

  • Flatworms (platyhelminths) – these include the trematodes (flukes) and cestodes (tapeworms).
  • Thorny-headed worms (acanthocephalins) – the adult forms of these worms reside in the gastrointestinal tract. The acanthocephala are thought to be intermediate between the cestodes and nematodes.
  • Roundworms (nematodes) – the adult forms of these worms can reside in the gastrointestinal tract, blood, lymphatic system or subcutaneous tissues. Alternatively, the immature (larval) states can cause disease through their infection of various body tissues. Some consider the helminths to also include the segmented worms (annelids)—the only ones important medically are the leeches. Of note, these organisms are not typically considered parasites.
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Although the term ectoparasites can broadly include blood-sucking arthropods such as mosquitoes (because they are dependent on a blood meal from a human host for their survival), this term is generally used more narrowly to refer to organisms such as ticks, fleas, lice, and mites that attach or burrow into the skin and remain there for relatively long periods of time (e.g., weeks to months). Arthropods are important in causing diseases in their own right, but are even more important as vectors, or transmitters, of many different pathogens that in turn cause tremendous morbidity and mortality from the diseases they cause.

Parasitic Infections:

Parasitic infections cause a tremendous burden of disease in both the tropics and subtropics as well as in more temperate climates. Of all parasitic diseases, malaria causes the most deaths globally. Malaria kills approximately 660,000 people each year, most of them young children in sub-Saharan Africa.