Upon hearing the word parasite, most people probably think of rather nasty creatures, such as the malaria parasites Plasmodium, trypanosomes, blood-flukes or fleas, which all cause serious diseases in humans. But parasitism can come in very different forms. A parasite is generally defined as an organism that benefits from exploiting a host belonging to a different species, be it for food, habitat or dispersal. Although the host is not killed by the parasite, it can suffer a substantial cost as its fitness is often significantly decreased (organisms that always kill their host, such as wasps that lay their eggs within caterpillars, are referred to as parasitoids). The typical parasite is much smaller than its host, highly specialised for its parasitic lifestyle, which normally involves a greatly reduced morphology and often suckers, claws or other attachment structures, and is able to reproduce very quickly indeed. This gives a substantial advantage in the evolutionary arms race with the host, which evolves adaptations to defend itself against the parasite. Different types of parasites can be distinguished. Ectoparasites (e.g. lice, ticks) live on the host, whereas endoparasites (e.g. most parasitic worms) live inside the host, either in or between its cells. Further examples include social parasites, which exploit interactions between members of social species such as ants or termites (see slavery in ants), or brood parasites such as many cuckoos, which have their offspring raised by host parents.

In terms of evolutionary convergence, interesting insights are provided by ectoparasites. Amongst the most successful of ectoparasites are insects, such as the well-known lice and fleas. Both parasitize numerous orders of vertebrates and show evidence for the independent acquisition of particular parasitic characters.

Some ectoparasites on fish are dinoflagellates, a large group of protists, most of which are free-living. Interestingly, some dinoflagellates have adopted parasitism independently a number of times, including the similar but unrelated fish parasites, Amyloodinium and Piscinoodinium. Even more remarkable than the ectoparasitic species is an endoparasitic dinoflagellate, Haplozoon praxillellae, which inhabits the intestines of a polychaete worm and shows a remarkable similarity to the tapeworms.

Many other parasites have evolved a worm-like design, too, such as the dicyemids and chromidinids. They typically inhabit the kidneys of cephalopod molluscs and share numerous features, from an unusual two-phase life cycle to specialised head and body shape. However, they evidently have evolved from separate branches of the tree of life (animals and protozoans, respectively), which indicates convergence between the two groups, driven by strong selective pressures for adaptation to a parasitic lifestyle.

A parasitic lifestyle involves reduction of some sort and an interesting example of convergent genome reduction can be found in some parasitic bacteria. They have independently lost genes for biosynthesis and other vital cellular functions, which are now undertaken by their hosts.

In higher animals, parasitism may take the form of brood parasitism. This has evolved several times independently in birds and not only have the parasites converged on certain adaptations needed to succeed as a brood parasite, but also their hosts have evolved similar defence mechanisms against parasitism. On the theme of host defences, several lizard genera have independently evolved mite pockets, folds or invaginations in the skin that provide ideal conditions for mite larvae to thrive in. It has been hypothesised that this represents a form of damage limitation, avoiding large-scale infection of the skin and superficial organs.

A great many plant species are parasitic. More than 4,000 species of flowering plant are known to derive at least part of their nutrients from other plants (or fungi that are associated with plants). Parasitism has evolved multiple times independently in dicotyledons and they have converged toward several types, such as root or stem parasites. Amongst the most famous parasitic plants are mistletoe, Rafflesia, dodder ("witches' hair") and broomrape.