Parasitism

Some living things live on or inside others.

They are called parasites. The host is the animal they live on. Parasites take food from the host. This can make the host feel sick.

Some live on the outside of the host. Others live deep inside the body. Some parasites use a third bug to travel. A mosquito can carry a tiny germ.

These tiny germs move from one host to another. Parasites are often much smaller than their hosts. It is a very close way to live.

Parasitism is a close way of living. One living thing is the parasite. The other is the host. The parasite lives on or inside the host. This relationship harms the host.

Parasites can live in different ways. Some are endoparasites. These live inside the host's body. Others are ectoparasites. These live on the host's surface.

Some parasites use a third party to travel. We call this a vector. A vector is a bug that carries a germ. For example, a mosquito can carry a germ to a person.

Other parasites move by being eaten. This is called trophically-transmitted parasitism. A small animal is eaten by a bigger one. The parasite then lives in the bigger animal.

Some parasites are very strong. They can be parasitic castrators. These parasites stop the host from having babies. This lets the parasite use the host's power to grow.

Parasitism is a special way that different living things live together. In this relationship, one living thing is called the parasite. The other living thing is called the host. The parasite lives on or inside the host for a long time. This way of living is helpful to the parasite, but it causes harm to the host.

Some parasites are very tiny, like single-celled protozoans. Others are much larger, such as mosquitoes, lice, or even vampire bats.

Scientists even describe parasites as predators that eat much less than a full prey item.

There are many different ways that parasites work. Some are endoparasites, which means they live deep inside the host's body.

Others are ectoparasites, which live on the outside surface of the host. Some parasites use a third party called a vector to move between hosts. For example, a mosquito can carry a germ from one person to another.

Other parasites use trophically-transmitted parasitism, which means they move by being eaten. A small animal might live inside a host until a predator eats it.

Humans have known about parasites for a very long time. People in ancient Egypt, Greece, and Rome knew about roundworms and tapeworms.

In 1681, Antonie van Leeuwenhoek used a microscope to see Giardia lamblia. Later, Francesco Redi described parasites like ticks and the sheep liver fluke.

Modern science for studying these creatures really grew during the 19th century.

Parasites use many clever tricks to survive and grow. Some are parasitic castrators, which stop a host from having babies.

This allows the parasite to use the host's energy to grow instead. Other parasites use host cues to find their way to a new home. These cues can be things like heat, moisture, or even smells.

Many parasites are also specialists. This means they can only live on one specific kind of animal.

We see the idea of parasitism in many places in our culture. In the 1733 poem "On Poetry: A Rhapsody," Jonathan Swift compared poets to tiny vermin.

Famous stories also use this idea to create excitement or fear. Bram Stoker wrote about the blood-drinking Dracula in 1897.

Even science fiction movies like the 1979 film Alien show parasitic creatures. These stories help us think about how these living things interact in the real world.

Parasitism is a close biological relationship between two different species. In this interaction, one organism acts as the parasite, while the other is called the host. The parasite lives on or inside the host for a period of time to gain resources. This relationship is harmful to the host, which may suffer from disease or reduced fitness.

Unlike predators that kill their prey immediately, most parasites do not kill their hosts right away. The entomologist E. O. Wilson described them as "predators that eat prey in units of less than one." This means they take only small portions of the host to survive.

Parasites use various biological strategies to exploit their hosts. One major way to classify them is by where they live. Endoparasites live inside the host's body, such as intestinal tapeworms.

Ectoparasites live on the external surface of the host, like lice or ticks. Some organisms, called mesoparasites, enter an opening in the host and remain partly embedded.

Parasites can also be classified by their life cycles. An obligate parasite must use a host to complete its life cycle. In contrast, a facultative parasite can live without a host if it chooses.

There are six major parasitic strategies used to exploit animal hosts. Parasitic castration is one extreme method where the parasite destroys the host's ability to reproduce.

For example, the barnacle genus Sacculina attacks the gonads of crabs. This diverts the crab's energy away from making offspring and toward growing the parasite. Another strategy is trophically-transmitted parasitism. In this case, the parasite is passed to a new host when the current host is eaten.

This often involves complex life cycles with both intermediate and definitive hosts.

Other strategies include vector-transmitted parasitism and direct transmission. Vector-transmitted parasites rely on a third party, like a mosquito, to carry them between hosts.

For instance, the Plasmodium parasite causes malaria and is moved by biting insects.

Direct transmission occurs when parasites move through simple contact without a third party. This includes many lice and mites that live on animals. Finally, parasitoidism involves insects that eventually kill their hosts, and micropredation involves small, brief attacks.

Parasites are often highly specialized to live on specific hosts. Many species are host-specific, meaning they can only survive on one particular type of animal. To find these hosts, parasites look for specific "host cues." These cues can be chemical or physical signals like heat, moisture, or exhaled carbon dioxide.

Some parasites are also divided by size into microparasites and macroparasites. Microparasites are tiny organisms like viruses or protozoans that reproduce inside the host. Macroparasites are larger, multicellular organisms that often reproduce outside the host body.

Human history shows a long awareness of these organisms. Ancient civilizations in Egypt, Greece, and Rome recognized parasites like roundworms. In 1681, Antonie van Leeuwenhoek used his microscope to observe the protozoan Giardia lamblia. Later, Francesco Redi provided detailed descriptions of parasites like ticks and the sheep liver fluke.

The formal study of parasitology grew significantly during the 19th century as scientific tools improved.

Parasitism also appears frequently in human culture and literature. In 1733, Jonathan Swift used the concept in a poem to satirize poets. In the realm of fiction, the idea is often used to create tension or horror. Bram Stoker's 1897 novel Dracula features a parasite that drinks blood.

Science fiction films, such as the 1979 movie Alien, also explore parasitic life forms. These cultural examples show how the concept of one being living off another is deeply embedded in our imagination.