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.
Entamoeba histolytica is a protozoan. A microscope is necessary to view this parasite. Credit: CDC. 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.
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:.
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:. But, parasites are eukaryotic organisms while pathogens except parasites and fungi are prokaryotic organisms. What is a Parasite — Definition, Features, Examples 2. What is a Pathogen — Definition, Features, Examples 3.
Ideally, parasites refer to organisms that are visible to the naked eye. Therefore, parasites are macroscopic organisms. Mainly, protozoans and helminths are considered as parasites. They are host specific. Hence, they only cause diseases only in specific organisms. Approximately, 36, species of protozoans have been identified as pathogens in other organisms.
On the other hand, about 70 species of protozoans and species of helminths are known as parasites in humans. Among them, about 90 species cause diseases in humans. On that account, not all parasites cause diseases in humans.
Plasmodium is one of the well-known examples of protozoan, which is a parasite in humans. A human louse is shown in figure 1. Tapeworm, roundworm, flatworm, fluke, and pinworm are examples of worm parasites including helminths.
Other than these two groups, lice and fleas are also considered as parasites in humans. They are called ectoparasites since they live on the host. The most characteristic feature of parasites is the high rates of reproduction.
A pathogen is an agent that causes a disease to its host. It can be a bacterium, fungus, virus, prion or parasite. The host organism can be a plant, animal or microorganism. Viruses are prime examples of a pathogen along with some bacteria, protozoa and fungi.
Some Bacteria, protozoa and fungi can be parasitic. But viruses aren't usually parasitic. Pathogenicity is a state of degradation of health. Parasitism is a mode of living for the microorganism. But as stated above, a clear difference between pathogen and parasite exists as a pathogen is the cause of a disease, whereas the same may not necessarily be the case for a parasite.
For example, Salmonella typhi is a pathogenic bacteria that cause typhoid fever, whereas, on the other hand, some bacteria can prevent colonisation by microorganisms such as Salmonella enterica - a bacteria that causes food poisoning, thus protecting the human body.
In another view, it can be easily considered that pathogens are the causative agents of any diseases, whereas parasites may or may not provide these causative agents after infection to the host.
As a parasite is dependent on its host to survive its life cycle, it may help the host to survive as well. But this is not necessarily true for every parasite.
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