Parasitism Definition

“Parasitism is defined as the relationship between different species in which one organism lives on or in the other organism and benefits from it by causing some harm.”

Table of Contents

What is Parasitism?

The word parasite is derived from the Latin form of the Greek word, meaning “one who eats at the table of another”. Parasitism is generally defined as a relationship between the two living species in which one organism is benefitted at the expense of the other. The organism that is benefitted is called the parasite, while the one that is harmed is called the host.

A few examples of parasites are tapeworms, fleas, and barnacles. Tapeworms are flatworms that are found attached to the insides of the intestines of animals such as cows, pigs, etc.. They feed on the host’s partly digested food, depriving it of the nutrients.

Also Read: Mutualism

Types of Parasitism

There are various types of parasitism and are classified based on their size, characteristics, interactions with their hosts and their life cycles.

Obligate Parasitism

This is the kind of parasitism in which the parasite is completely dependent on the host to complete its life cycle. Obligate parasites cannot survive without the host. Therefore, they do not severely harm the host. Fungi, bacteria and viruses exhibit obligate parasitism. For eg., head lice, when removed from the human scalp, dies.

Facultative Parasitism

In this kind of parasitism, the parasite is not completely dependent on the host to complete its life cycle and can survive without the host. A nematode species Strongyloides stercoralis is found free-living but causes a disease strongyloidiasis when it infects humans.


The parasites that live outside the body of the host exhibit ectoparasitism. For eg., lice and ticks


Parasites that live inside the body of a host exhibit endoparasitism. For eg., hookworms and nematodes.


The parasites that enter the external openings of the host exhibit mesoparasitism.

Examples of Parasitism

There are many examples of parasites, as it is a type of interaction between the two species. Listed below are some of the common examples of parasitism.

Parasitism in Humans

The organisms that parasitize humans include fungi, leeches, lice, viruses, protozoa, tapeworm, etc. Few organisms such as Helminthes live inside the intestine of the host and causes several infectious diseases, such as jaundice, malnutrition, diarrhoea, etc. All the infections are caused by viruses and bacteria.

Read more: Saprophytes

Parasitism in Plants

Small green insects called aphids, parasitize plants by eating their sap. Several types of fungi parasitize crops ad spoil fruits, vegetables and food grains. The parasitic plants contain modified roots called haustoria which connect to the host xylem or phloem and drain it of nutrients and water.

Parasitism in Insects

Parasitism is very common in insects. Entomophagous parasites attack larva and young insects. A few insects deposit their eggs within the body of the larva of other insect species. When the eggs hatch, the young one eats the larva and derives nutrition from it.

Also Read: Parasites and Symbiosis

It is evident that parasitism has very little to offer in terms of benefits but it helps in maintaining the biodiversity of the ecosystem and controls the population to a large extent. Parasite-host interactions may be important at times.

For example, parasitoids controlling the body temperatures of their hosts in order to ensure the survival of their offspring and host choice in fleas being controlled by the off-host environment.

To know more about what is parasitism, its definition, types and parasitism examples, keep visiting  BYJU’S Biology website or download BYJU’S app for further reference.

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