There are a lot of different types of parasite out there. In order to better understand and summarize them, we had decided to make two comprehensive tables including all the main parasite groups. Some examples of species from the different groups are also found in these tables, as well as some important features related to them. A striking and impressing feature shown in these tables, especially in the one with the helminth groups, is the very large number of estimated infected people worldwide. However, for many of these parasites, the World Health Organization estimates that these numbers represent in fact an underestimation of the real situation, mainly because these diseases occur more frequently in developing countries where health facilities are less efficient. In fact, in many of these countries, medicines for parasitic diseases are available either on the black market or over the counter in the drugstore. This means that it is possible to get an effective treatment if you think you are infected with a parasite without even consulting a doctor and having a proper diagnosis. Furthermore, many parasitic diseases can remain asymptomatic for a long time, meaning that the person does not have any idea that he or she is infected. These infected people are then not represented in the official statistics. As such, for many parasites, the real number of infected people worldwide is probably higher than the official estimation. Finally, it is important to note that these tables do not include all the possible parasite species, but only notable members of these groups as examples of the more frequent or commonly known parasites in these groups. Consequently, these tables summarize the known data about these types of parasites and they do not cover all the information available on these groups.

It is also interesting to note that it is possible to observe some disparities between English-speaking countries about the common popular name given to some parasites. These differences can sometimes be very confusing. For example, the roundworms from the genus Enterobius are commonly known as pinworms in North America. However, these parasites are known as threadworms in the United Kingdom and in Australia. To complicate the matter a little more, the term pinworm in the United Kingdom refers rather to another parasite from the roundworm group, namely Strongyloides species. However, in North America, this type of parasites is the one that is known as threadworm.

The pictures presented in these tables are showing examples of the parasite species chosen to represent the parasitic group involved. As shown, the helminths are usually visible with the naked eyes and their pictures are then taken using a normal camera. As such, they can be considered as some kind of small animals. It is interesting to note that some helminths are able to become very long, as for example the Taenia species that are able to reach a length of 8 meters. On the other hand, the picture of Schistosoma was magnified under the microscope because, although they are theoretically visible with the naked eyes, they are still quite small with a length ranging between 6 and 17 millimeters. This picture of Schistosoma is quite remarkable as it highlights a feature that is unique within the group of the flatworms. In fact, this parasite has a distinct sexual dimorphism, meaning that it has a different form for the male and the female. In this picture, the more robust one is the male form while the more slender one is the female form. As shown in the picture, the adult female always resides in the kind of sexual canal of the adult male. Finally, the picture of Necator americanus was also magnified in order to better show its mouthpart that is responsible to hook the parasite within the intestinal walls.

Protozoan parasites are microscopic and then not visible with the naked eyes. Consequently, their pictures are all magnified under the microscope. This characteristic highlights the main difference between the protozoan parasites and the helminths, as the former ones are microscopic organisms composed of only one cell and the latter ones are some kind of small visible with the naked eyes animals composed of many cells. Interestingly, the characteristic features related to their motility mode that distinguish each group of protozoan parasites are clearly visible on these pictures. As such, if someone looks closely at the picture of the ciliate Balantidium coli, it is easy to see the cilia, the structures looking like little hairs, all around the parasite. Similarly, the flagella, the structures resembling longer hairs are also visible in the pictures of the flagellates. It is interesting to note on these pictures that some flagellated parasites, such as Trichomonas vaginalis and Giardia intestinalis, possess many flagella while others, such as Leishmania and Trypanosoma species, have only one flagellum. The characteristic pseudopods of the amoeba, represented by some kind of cell projections or extensions, are also visible, especially in the picture of Acanthamoeba species. Finally, the pictures of the Sporozoa does not shown any organs related to motility as their mature forms are non-motile. Furthermore, the pictures of the coccidian parasites show really well the round shape that gives them their name, as the prefix cocci- usually means having a round shape.

However, for all the species shown in these tables, it is interesting to keep in mind that the picture represents only one form of the parasites. However, some parasites can present themselves under different forms throughout their life cycle. Furthermore, many parasites have a cyst or an egg form that is directly involved in the infection process.

Sources: Centers for Disease Control and Prevention, World Health Organization; All the parasites’ pictures are free of copyright and had been found in the DPDx web site developed by the Centers for Disease Control and Prevention’s Division of Parasitic Diseases and Malaria (DPDM) in the image gallery specific for each shown parasitic species.

Common Name	Thorny-,or spiny-headed worms	Roundworms	Whipworm	Hookworms	Tissue flukes (trematodes)	Blood flukes (trematodes)	Tapeworms (cestodes)
Group Name	Acantocephalins	Nematodes (nemathelminths)			Flatworms (platyhelminths)
Example of Species	Moniliformis moniliformis	Ascaris lumbricoides	Trichuris,trichiura	Necator,americanus	Fasciola,hepatica (sheep liver fluke)	Schistosoma, species	Taenia, species
Disease Name	Acanthocephaliasis	Ascariasis	Trichuriasis	Necatoriasis	Fascioliasis	Schistosomiasis or,bilharziasis	Taeniasis and,cysticercosis
Estimated,number of infected people worldwide	Very few	More than one billion	A little less than one,billion	700 million (both,hookworms)	Between 40 and 100,million (all tissue flukes)	More than 260 million	More than 100 million, (only for Taenia species)
Notable characteristics	Very rare; Humans are, accidental hosts	One of the most,frequent human parasites	Have important,socio-economical impacts in developing countries			Second parasitic,disease with the greatest economic impacts after malaria
Modes of transmission	Contaminated food with, raw beetles or cockroaches	Contaminated soil/ Contaminated,food or water	Contaminated soil/ Contaminated,food or water	Contaminated soil/ Contaminated,food or water	Contaminated food,growing within water	Skin contact with,contaminated water	Contaminated food
Main symptoms	Gastrointestinal	Asymptomatic/Gastrointestinal/Other tissues	Asymptomatic/Gastrointestinal	Gastrointestinal	Asymptomatic/ Target,specific tissues	Gastrointestinal/Urinary tract	Asymptomatic/Gastrointestinal
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Common name	Ciliates	Flagellates				Amoeba		Sporozoites	Coccidian parasites
Group name	Ciliophora	Mastigophora				Sarcodina		Sporozoa (or Apicomplexa)
Example of species	Balantidium coli	Trichomonas vaginalis	Giardia intestinalis	Leishmania species	Trypanosoma species	Entamoeba histolytica	Acanthamoeba species	Plasmodium species	Toxoplasma gondii	Cryptosporidium species
Disease name	Balantidiasis	Trichomoniasis	Giardiasis	Leishmaniasis	Sleeping sickness (African trypanosomiasis) or Chagas disease	Amoebiasis	Acanthamoeba keratitis, disseminated Acanthamoeba infection or granulomatous amoebic encephalitis	Malaria	Toxoplasmosis	Cryptosporidiosis
Estimated number of infected people worldwide	Between 1.4 and 7 million	More than 170 million	One billion	More than 12 million	Between 6 and 7 million	Up to 50 million	Few thousands (only 400 known cases of granulomatous amoebic encephalitis)	200 million	Between 2.3 and 3.5 billion	Between 250 and 500 million
Notable characteristics	Largest protozoan parasite able to infect human; Humans are accidental hosts	The most frequent curable sexually transmitted disease; Human is the only known host	One of the most frequent parasitic infections; Resolved usually by itself except in severe cases	Common co-infection for people living with HIV			The most frequent protozoan parasite found in soil; most people came in contact with it, but resulting illnesses are rare	The parasitic disease with the greatest economic impacts	One of the most common human infection worldwide	Resolved usually by itself except in severe cases
Modes of transmission	Contaminated soil/ Contaminated food or water	Sexual intercourse	Contaminated food or water	Insect bites (sandfly)	Insect bites (tsetse fly) or contact with insect faeces (kissing bug)	Contaminated food or water	Contaminated contact lenses/ Contaminated soil in contact with skin lesions or nostrils	Insect bites (female Anopheles mosquito)	Contact with soil contaminated with cat’s faeces/ Cat’s litter box	Contaminated food or water/ Recreational waters
Main symptoms	Asymptomatic/ Gastrointestinal	Asymptomatic (mostly in men)/ Genitals and urinary tract	Asymptomatic/ Gastrointestinal	Skin/Internal organs	Neurological/ Internal organs	Asymptomatic/ Gastrointestinal/ Internal organs	Eyes/Internal organs/Brain and spinal cord	Wide range of non-specific symptoms	Mostly asymptomatic	Asymptomatic/ Gastrointestinal
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