There are two different types of parasites: Protozoans and Helminths, we will cover more on the topic in other articles this one should give you a basic idea.
The first type of parasites that can live inside the human body is called protozoan parasites. In respect with some exceptions, this group includes different species of motile unicellular eukaryotic parasites, some of them being pathogenic for humans. This description means that they are composed of only one cell with a nucleus, which is the structure containing most of the genetic material of the cell, and one or more mitochondria, which are the organelle responsible for the generation of energy. Protozan parasites are usually very small (somewhere between 10 and 50 micrometers) and visible only under the microscope.
There are four types of protozoan parasites that are classified according to the way they move. The Sarcodina, like Entamoeba, are the amoeba group that moves using pseudopods extension and retraction altering their cell shape. The Mastigophora, such as Trypanosoma and Leishmania, move using flagella whereas the Ciliophora, like Balantidium, use cilia. Flagella and cilia are both used to propel the cell in its environment and are structurally similar but can be distinguished by their length and function. The last group is the Sporozoa, such as Plasmodium and Cryptosporidium, which are not motile anymore when they are in their mature stage.
It is interesting to know that some protozoans, like Entamoeba, have a life cycle alternating between proliferative and cyst stages. The proliferative stage is the active form of the parasite meaning that the parasite feeds itself and reproduces a lot. The cyst form allows them to survive for a long period, even many years, without food or oxygen, in hostile environment, such as high temperature or chemically contaminated. This dormant form is still infectious and the parasite will come back to the proliferative stage when encountering a suitable host, for example when ingested with contaminated food or water. Other protozoan parasites, like Plasmodium, have a life cycle where they alternate between one form in their insect vector and another form when they reach their human host.
Protozoan parasites can cause a wide range of different diseases. Some of the most commonly known include malaria, which is caused by a parasite from the Plasmodium genus, and amoebic dysentery, which is an intestinal inflammation caused by the amoeba Entamoeba histolytica.
Finally, it is noteworthy to mention that even though the protozoan appellation is now less common in fundamental sciences because of the newest taxonomic technology based on DNA sequencing proving that most of these organisms are not really related genetically, it is still a very common term in medicine.
The second type of parasites infecting humans is called helminths, but is more commonly known as worm parasites. As opposed to protozoans, this group is composed of larger multicellular organisms that can usually be visible with the naked eye. The general name of the infectious disease caused by helminths is helminthiasis. The majority of the helminths lives in the intestine and are therefore often called intestinal worms. Schistosoma, which is a genus of trematodes causing a disease commonly known as blood-flukes, constitute one of the exceptions because it lives in the blood vessels of its host.
As intestinal parasites, helminths are generally transmitted by direct contact with faeces from an infected animal or human, by contact with contaminated soil, or by ingestion of contaminated food or water. They can generally live for years in their human or mammalian host by modulating their host’s immune system even though their mature form is not able to multiply anymore in human.
As a mean of reproduction, all helminths produce eggs that are able to survive harsh environmental conditions for a long period of time, ranging from many months in soil and water to several years in sludge and faeces. These eggs are essential for their reproduction cycle that continues when larvae hatch from the eggs. Depending on the species of helminths, the larvae can hatch outside or inside the host in order to pursue the life cycle. Consequently, either helminth eggs or larvae depending of the type of worm involved can infect humans.
Like the protozoan parasites, there is no scientific consensus on the taxonomic classification of helminths. However, they are generally classified in three different groups according to their morphologic characteristics. These groups are the platyhelminths or flatworms including the cestodes and trematodes, which are respectively known as tapworms and flukes because of their flat bodies, the nemathelminths or nematodes, commonly known as roundworms because of their round shape, and the acanthocephalins, commonly known as thorny-headed worms and thought to be an intermediate form between cestodes and nematodes.
It is interesting to mention that some scientists note that people living in undeveloped or developing countries where helminths are widespread have a very low prevalence of autoimmune diseases compared with the high prevalence of these diseases seen in developed countries. The main hypothesis explaining this finding is that helminths (when they stay at a low proliferative stage), in interaction with bacteria already colonising the gut, are modulating the immune response and though confer a protection against autoimmune diseases. Consequently, there is now a novel area of research focusing on helminths as a treatment for autoimmune diseases, such as Crohn’s disease, celiac disease, atherosclerosis, ulcerative colitis and multiple sclerosis. However, these experimental treatments can have some side effects, such as fatigue and gastrointestinal issues.
Sources: Center for Disease Control and Prevention; World Health Organization