Symbiosis is defined as the co-evolved intimate relationship between members of different species. It comes in many flavours, including the one-sided and abusive parasitic relationship and the more reasonable mutualism where both parties benefit from the relationship. A third type, commensalism, is a relationship where one species benefits from the other one-sidedly, but without harming it.


Parasites are highly specialised organisms that derive food from their host, at its expense. Examples include the tapeworm (Taenia) as well as lice (Pediculus), both body and hair lice.

Tapeworms reside in the small intestine where they can conveniently tap into the host’s nutrients, and so interfere with the normal absorption of the host’s nutrients into its bloodstream, thereby depriving the host to potentially dangerous levels of malnutrition and other side effects such as anaemia and fatigue.

Lice live on the body, such as pubic lice or on eyelashes, or the scalp in the case of head lice, and feed on the host’s blood. Sensitivity to lice saliva causes itchiness, and some lice can be vectors (carriers) of dangerous infectious agents such as Epidemic typhus and Trench fever.

Tapeworms such as Taenia solium, the pork tapeworm, can be more than a couple meters long, and has a specialised head that helps it attach to the small intestine of the host. It is hermaphroditic as its segments each have full reproductive systems. These segments of its body are called proglottids.


Malaria is transmitted via a vector which carries it without being affected, before passing it onto the final host of the parasite. In this case it is carried by the mosquito which transfers it via its bite in saliva. As such, the mosquito is the secondary host while the human is the primary host. The primary host is where the parasite actually reaches maturity and reproduces.

Symptoms upon infection include fever, headache, vomiting and fatigue…

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