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Biology Notes for Class 12 – What is a Parasite? – It’s Types & Characteristics

Biology Notes for Class 12 - What is a Parasite? Types & Characteristics, NCERT Solutions for Class 12 Biology,

Biology Notes for Class 12 – What is a Parasite? It’s Types & Characteristics

What is a Parasite? Types & Characteristics :- 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”

A parasite is an organism that lives in/on 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. 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 parasites that cause diseases, they are :-

  • PROTOZOA (cause protozoan infection) – 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 gives rise to infections from just a single organism. Transmission of protozoa that live in a human’s intestine to another human typically occurs through a fecal matter or through 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:

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


  • HELMINTHS (causes helminthiasis) – 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:


  1. Flatworms (platyhelminths) – these include the trematodes (flukes) and cestodes (tapeworms).
  2. 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.
  3. 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.


  • ECTOPARASITES- 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).

Watch videos on the same for better understanding and learning, click on Biology Class 12

Protozoa and helminths are usually endo-parasites (usually living inside the body of the host), while ectoparasites usually live on the surface of the host. Occasionally the definition of “parasitic disease” is restricted to diseases due to endo-parasites. Parasitism is the relationship between two organisms in which one benefits at the expense of the other, sometimes without killing the host organism.

Endo-parasites may be either intercellular (inhabiting spaces in the host’s body) for example, worms/ helminths, or intracellular (inhabiting cells in the host’s body). Intracellular parasites—such as bacteria or viruses—often rely on a third organism, known as the carrier, or vector, to transmit them to the host. Malaria, which is caused by a protozoan of the genus Plasmodium transmitted to humans by the bite of an Anopheles mosquito, is an example of this interaction.A definitive or primary host is the one in which a parasite, reaches maturity and, if possible, reproduces sexually. … A secondary host or intermediate host is a host that harbours the parasite only for a short transition period, during which (usually) some developmental stage is completed.

A parasitic disease is also known as parasitosis, and the study of parasitic diseases is called parasitology.

In Biology, parasitism is a non-mutual relationship between species, where one species, the parasite, benefits at the expense of the other, the host. Traditionally parasite primarily meant an organism visible to the naked eye, or a macroparasite (such as a helminth). Microparasites are typically far smaller, such as protozoa, viruses, and bacteria.

Study in detail Protozoa, Helminths, and Ectoparasites with animated videos click on Biology notes Class 12.

Unlike predators, parasites typically do not kill their host and are generally much smaller than their host, and often live in or on their host for an extended period. Parasites show a high degree of specialization and reproduce at a faster rate than their hosts. Classic examples include interactions between vertebrate hosts and parasites like- tapeworms, flukes, the Plasmodium species, and fleas.

Parasites reduce host biological fitness by general or specialized pathology, from parasitic castration and impairment of secondary sex characteristics to the modification of host behaviour. Parasites increase their own fitness by exploiting hosts for resources necessary for their survival, in particular transmission from one host to other.

Warning signs that your Body is Full of Parasites

Biology Notes for Class 12 - What is a Parasite? Types & Characteristics, NCERT Solutions for Class 12 Biology

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