Parasitism is a kind of biological relationship between two different species of living organisms. Parasitism is widespread and almost all living organisms can be a host, meaning that it is infected by a parasite. Even parasites can sometimes be hosts for another parasite. Usually, this kind of relation has more or less important negative impacts on the host but is beneficial for the parasite. Actually, the parasite needs the host to feed, to get a shelter and/or to reproduce. Interestingly, some parasitic relationships can last a very long time with few negative effects on the host. However, some parasites can also kill their host more or less quickly, even though it is not their primary goal.
Parasites have evolved to be quite specific, meaning that they can infect only one or a few host species. Some parasites can also change host depending on which stage they had reach in their life cycle. In that case, the parasite infects a host at one point of its life cycle (for instance, reproduction or maturation) and another host for the next stage in its life cycle. Consequently, animals or insects can be intermediate hosts at one point in the life cycle of a parasite infecting usually humans as its definitive host. In some case, parasites can have reservoir host that is asymptomatic and harbour them indefinitely. The reservoir host can be used to infect directly or via a vector an intermediate or definitive host.
Parasites and their host have usually evolved together for hundreds or thousands of years. This phenomenon is called coevolution and allows the parasites to elaborate more sophisticated strategies to infect and live inside their host. These strategies are commonly referred to as host-parasite interactions. Coevolution also explains the host specificity of parasites.
A wide variety of parasites exist in nature, but only few of them are able to infect humans. The scientific branch studying these ones is called medical parasitology. Even though some other organisms, such as bacteria or fungi, can establish a kind of parasitic relationship with humans, medical parasitology usually refers strictly to protozoans and helminths. These two categories are composed of endoparasites meaning that they live inside the human body. These parasites caused parasitic diseases that are part of the larger group of infectious diseases. It is interesting to note that ectoparasites, such as lice and fleas, which live on outside the human body, can sometimes be included in a much larger definition of parasitic diseases. However, the strict definition of this type of infectious diseases is usually restricted to endoparasites.
As parasites are often transmitted by insect bites and contaminated food and water, they are generally more prevalent in tropical and subtropical regions. However, parasites are not considered as contagious diseases because direct human-to-human transmission is rare and usually limited to lack of hygiene or direct contact with faecal matter.
Since there are a lot of different parasite species infecting humans and that they affect mostly people in undeveloped or developing countries with inadequate health infrastructures, it is very difficult to estimate the global burden of parasitic diseases. However, the World Health Organization estimates that more than one billion people, which is one seventh of the global population, suffer from at least one of the seventeen most prevalent infectious diseases collectively called Neglected Tropical Diseases in undeveloped or developing countries, the majority of them being parasitic diseases.
Vaccines are not yet available against parasites despite extensive research efforts in this area. For some parasites, preventive treatment can be available for people travelling in endemic countries, such as the well-known Malarone pills that are prescribed to people travelling in a zone where malaria infection is prevalent. Other prevention means includes sleeping under net that prevent insect bites, insect vector population control and improvement of hygiene.
Most of the parasites infecting humans can be treated with antiparasitic drugs. However, like with the antibiotics, the main problem nowadays with antiparasitic drugs is resistance. Drug resistance is widespread among microorganisms and it means that they evolve by different ways to survive and proliferate despite the administration of the drug. This phenomenon limits a lot the treatment of some parasites, especially in undeveloped or developing countries that cannot get the newest expensive drugs on the market. Furthermore, some antiparasitic drugs have a certain level toxicity against human organs that can lead to major side effects. More scientific researches are needed in order to find more effective and less toxic antiparasitic drugs.
Sources: World Health Organization; ARC/NHMRC (Australian Research Council/National Health and Medical Research Council) Research Network for Parasitology