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Provided one can get away with it, stealing something is easier than making it oneself or earning money to purchase it. Therefore, it is unsurprising that so many organisms have adopted a parasitic lifestyle to some extent. If one takes the view that the main purpose of an organism’s existence is to transfer as many of its genes as possible into the next generation, then all organisms should maximise their reproductive output. However, an organism must trade the costs of reproduction against other activities such as finding food and then digesting and absorbing it, finding a mate, and protecting itself against competitors, predators, and the environment. By living upon or within a host, a parasite can reduce many of these ‘other costs’ and thereby devote more of its time and energy to reproduction. Most parasites stay in association with their host for the duration of a life cycle stage, and therefore, having located and infected their host, the need for sensory apparatus and locomotion are reduced because the parasite has access to a guaranteed food source. This guarantee also means that the parasite does not have to extract as much energy as possible from each ‘unit of resource’. Instead, it can afford to be wasteful, and many parasites have reduced metabolic pathways. Furthermore, there is no need to lay down metabolic reserves beyond those required for the next life cycle stage. Parasites rarely need well‐developed food gathering apparatus and, in some cases, such as the tapeworms, they have dispensed with a mouth and gut altogether, relying on nutrients being absorbed across the body wall.

Because parasites live within or upon their host, they have less need to maintain body surfaces and behaviours that protect them from desiccation, heat, cold because this is done by the host. Similarly, the parasite is to a large extent protected from predators and pathogens, because these must overcome the host’s immune system before locating the parasite. Even ectoparasites receive protection to some extent because hosts cannot always distinguish between a predator attempting to take a bite out of them from an animal solely interested in removing a flea or louse.

A parasite will be transported wherever the host goes and therefore the limits of its dispersal depend upon the dispersal powers of its host, coupled with whatever other special needs the parasite must complete its life cycle (e.g., the presence of a suitable vector or environmental conditions). Consequently, a parasite does not have to devote energy to dispersal.

Table 1.1 Summary of advantages and disadvantages associated with the parasite lifestyle.

Advantages	Disadvantages
Once host located, no need for further searching	Extreme host specificity can increase vulnerability to extinction
Food permanently available
Limited requirement for complicated food capturing mechanisms	Must locate at optimal site on/in host to ensure food/survival
Reduced need for food processing
Protection from environmental extremes	Must adapt to host’s internal physiological environment (internal parasites only)
Protection from predators and diseases	Must overcome host’s immune defences
Reduced need for dispersal because host (+ vector) carries the parasite.	Spread limited by host’s geographic range
Can devote larger proportion of energy intake to reproductive output than a free‐living organism	Transmission can be extremely risky and most offspring die before establishing in a new host

If the benefits of parasitism are so enormous, this therefore begs the question why there are not more highly specialized parasites and why parasitism tends to be extremely common among some groups of organisms but rare among others. For example, there are comparatively few parasitic higher plants, Lepidoptera, or vertebrates.

Any would‐be parasite must first overcome the putative host’s immune defences and adapt to its internal physiological environment: this involves many physiological modifications, and therefore most parasites are host specific. However, host‐specificity places the parasite in a difficult situation because its existence then becomes dependent upon that of its host. Should the host become extinct, then its parasites will follow suit unless they are able to infect other organisms. Furthermore, for the individual parasite, finding hosts is seldom easy. Although many parasites produce huge numbers of offspring, the chances of any one of them managing to locate a suitable host, establishing an infection, and reproducing successfully are extremely small. The advantages and disadvantages of the parasite lifestyle are summarised in Table 1.1.