Different parasites have different life cycles, but they share some similarities. Parasites usually have one or more hosts in which they can take shelter, change form and reproduce themselves. They can have a definitive or final host where they reproduce themselves and an intermediate host where they gets shelter and change form before being able to infect their definite host. In that case, infection of one or more intermediate hosts is required for the maturation process of the parasite. If the parasite needs a host to fulfill its life cycle, it is called an obligate parasite.
Most of the parasites infecting human are obligate parasites having human as their definitive host. A small proportion of parasites, such as the protozoan parasites and Trichomonas vaginalis, have human as their only known host. In that case, it could be because they are not able to infect another species than human or it could also be because scientists have not found yet any intermediate host. However, the majority of the obligate parasites infecting human have one or more intermediate hosts. These hosts are usually some specific animals or insects. Of course, vector-borne parasites have the insect vector as one of their intermediate hosts or as their only intermediate host. For example, Leishmania has the sandfly as its intermediate host and Plasmodium has the female Anopheles mosquito as its intermediate host.
Animals can also be considered as intermediate host. For example, pork is an intermediate host for Trichinella spiralis and pork and beef are intermediate hosts for Taenia, depending on the species. This is the reason why it is possible to acquire these parasites while eating raw or not enough cooked meat. It is also possible for human to acquire parasites by contact with contaminated faeces from an intermediate host. For example, cats act as intermediate hosts for Toxoplasma gondii and contact with contaminated cat’s faeces transmit the parasite.
A small proportion of parasites has more complex life cycles and is able to infect a larger number of species. For example, the helminth Diphyllobothrium has some crustacean like copepod as its first intermediate host. Then, some small freshwater fishes that can be in turn eaten by larger fishes, such as trout and perch, eat these crustaceans. Eating raw or undercooked fishes contributes to infect human, but also many other types of mammals.
Some intermediate hosts are also more frequent than others. For example, freshwater snail is an intermediate host for many helminth species. As such, most of the trematodes, commonly known as flukes, have freshwater snails as their first or only intermediate host. Freshwater snails released the parasite within the water and, depending on the species involved, it can infect a second intermediate host, which is usually a fish, or it can infect directly its definitive host, which is an animal or human, depending on the species involved.
On the other hand, if the parasite is able to complete its life cycle without infecting a host, it is called a facultative parasite. It is the case of the free-living amoebae group including very rare but life-threatening parasites, such as Acanthamoeba, Balamuthia mandrillaris and Naegleria fowleri. This type of parasites can live as free organisms in nature and sometimes but rarely infects a host.
Moreover, it happens that a parasite accidentally infects a host in which it is unable to efficiently complete its life cycle. In that case, the host is called an accidental host for this type of parasites. This is the case of some uncommon parasitic diseases in human, such as the Acanthocephalins species of helminths commonly known as thorny-headed worms (or spiny-headed worms) and the protozoan parasites Balantidium coli.
Finally, this post only gives some general examples of common parasitic life cycles. This is also very simplified as some parasites can have a very complex life cycle involving many transformations, known as lifecycle stages, before infecting their definite host. However, as each parasite has its own specific life cycle, it is better to look specifically for the life cycle of your parasite of interest.
Source: Centers for Disease Control and Prevention.
