What is a Parasite?

A parasite is an organism that lives in the body of another organism and obtains food at the expense of the host. There are two kinds of parasites they are internal and external. An internal parasite lives inside another organism and is known as endoparasite. An external parasite lives on the body surface of the host and is known as an ectoparasite.

What is parasitic infection?

The infection caused or transmitted by parasites is referred to as parasitic infection. Many parasites do not result in infection, but few parasites reproduce and invade the host's organ system making them sick and thereby resulting in infection. Parasitic infection mainly occurs in plants and animals. Parasitic infections majorly occur in the tropical or subtropical region of the world.

Causes of parasitic infections

Three organisms are mainly responsible for causing parasitic infections can. These are protozoa, helminths, and ectoparasite.


Protozoa is a single-celled eukaryotic microorganism that can live and multiply inside the host. Protozoa live freely and may be parasitic or symbiotic. They are motile have locomotive organelles, for example, flagella and cilia for movement. They are classified based on their motility and method of reproduction. They are classified mainly into four types: flagellates, ciliates, Sarcodina, and sporozoites. The infection caused by Giardia lamblia (parasitic protozoa) results in giardiasis, which is the infection of the small intestine.

The diagrammatic representation of protozoan is shown in the figure.
CC BY-SA 4.0 | Image credits: https://commons.wikimedia.org Frank fox


Malaria is a protozoan disease caused by Plasmodium spp. The species involved in malaria are Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, and Plasmodium malaria. Usually, malaria spreads through the biting of female anopheles mosquitoes when they bite the infected individual and then bite the healthy individual and transfer the parasite to them. The major symptoms of malaria include chill, fever, headache, sweating, etc. Sometimes people have abdominal pain and diarrhea.

Life-cycle of malaria

Malaria begins when female anopheles mosquitoes inoculate plasmodial sporozoites at the time of feeding. The small motile sporozoites are injected during the phase of probing as the mosquito searches for avascular space before aspirating blood. After injection, they enter the circulation either directly or via the lymph channel and rapidly target the hepatic parenchymal cell. Each sporozoite enters into the hepatocyte and there begins with a phase of asexual reproduction. This stage lasts on average between 5.5 and 15 days before the hepatic schizont ruptures to release merozoites. Malaria is diagnosed by blood tests or rapid diagnostic kits.  Malaria is prevented by avoiding mosquito bites, taking preventive drugs, etc.

The diagrammatic representation of the Life cycle of Malaria is shown in the figure.
CC BY-SA 4.0 | Image Credits: https://commons.wikimedia.org | Bbkkk


The helminths are visible to the naked eyes in their adult form. In their adult stage, they do not multiply in the host. They instead lay eggs which are released into the environment through fecal matter. Helminths are mainly of three types: flatworm, roundworm, and thorny-headed worms.

The structures of helminths is shown in the figure.

Hook-Worm Infection

Hookworm is a parasite that can infect people of all ages. It enters the body through the cut on the skin and can lead to several complications. Hookworm can spread when a person who has the infection defecates in the soil and when people use human feces as a fertilizer. The eggs in the feces can hatch into the larvae, which can be ingested with raw vegetables leading to the spread of infection. Fever, coughing, diarrhea are the symptoms. In the microscopic examination, the stool of the infected person shows the presence of eggs.

Tapeworm Infection

Tapeworm infection is caused by ingesting food or water contaminated with tapeworm eggs or larvae or by eating undercooked meat, such as beef and pork. Upon ingestion, the tapeworm larvae can move out of the intestine and can form larval cysts in the tissues or organs. Eating raw or undercooked meat helps in the transmission of tapeworm resulting in tapeworm infection. Cysticercosis is the infection of the muscle tissues and brain by the tapeworm Taenia solium.


Ectoparasites are the organisms that live on the surface of the host and feed on their blood which provides the nutrition required for survival. In humans, ectoparasites are comprised of two major animal groups: parasitic arachnids and parasitic insects. Parasitic arachnids include ticks and mites. Ectoparasitic insects are exemplified by mosquitoes, tsetse flies, fleas, and lice.

Who is at risk of parasitic infections?

•  Individuals that live or travel in the tropical or subtropical region of the world.

•  Areas that lack a clean supply of drinking water.

•  Individuals working with soil or other surfaces can come in contact with the feces consisting of the helminths.

Diagnosis of parasitic infections

Parasitic infections are diagnosed by various methods, such as:

•  Blood test.

•  Fecal examination under the microscope.

•  An endoscopy or colonoscopy. This test may be ordered if the results of a stool exam are inconclusive.

Prevention of parasitic infections

Certain measures help in the prevention of infection caused by a parasite, such as:

•  Food should be cooked properly.

•  Avoiding insect bites.

•  Avoiding contact with contaminated food, water, and soil.

•  Keep good personal hygiene.

Context and Applications

The topic is important for professional exams for both school level, undergraduate and postgraduate levels, mainly for bachelors and masters in epidemiology, and bachelors and masters in microbiology.

Practice Problems

Question 1: Hookworm lives inside the intestine and is involved in _______.

1. Consuming feces

2. Sucking blood

3. Digestion of cellulose

Answer: Option 2 is correct.

Explanation: Hookworms live inside the intestine and with the help of the sucker it sucks the blood.

Question 2: Disease, damage, and death are caused due to _________.

  1. Parasitic infections
  2. Neutral relationship
  3. Symbiotic relationship

Answer: Option 1 is correct.  

Explanation: Infection with parasites causes disease and death. Parasitic infections are a major issue in the world's tropical and subtropical regions. Malaria is one of the deadliest parasitic diseases. Parasitic infections can cause disease, damage, and death in human beings.

Question 3: The parasites that live inside the host body are called ________.

  1. Ectoparasite
  2. Endoparasite
  3. Digestion parasite

Answer: Option 2 is correct. 

Explanation: The parasite that live inside the host body and absorb nutrients are known as endoparasites. The parasite that is present on the surface of the host is called an ectoparasite.

Question 4: Rounded head like structure in tapeworm is called as_________.

  1. Neck
  2. Joint
  3. Scolex

Answer: Option 3 is correct.

Explanation: Scolex is attached to the tapeworm intestinal wall and neck.

Question 5: Which of the following is not caused by liver fluke in animals?

  1. Diarrhea
  2. AIDS
  3. Eosinophilia
  4. Anemia

Answer: Option 2 is correct. 

Explanation: Liver fluke causes inflammation of bile ducts, anemia, diarrhea, eosinophilia, and liver rot which may lead to the death of the animal.

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