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The term ‘harm’ is often employed when describing interactions between organisms but is particularly pertinent to any discussion of host: parasite relationships. Unfortunately, harm is a difficult term to define and is not always easy to measure. For example, parasites are usually much smaller than their host and a single parasite may have such a minor impact that its effect on the physiology and well‐being of the host is too trivial to measure. By contrast, a large number of the same parasite could cause serious illness or even death. Similarly, a low parasite burden may have little impact upon a healthy, well‐nourished adult host, but the same number of parasites infecting an unhealthy, starving young host may prove fatal. Consequently, harbouring a pathogen (being infected) and expressing the signs and symptoms of being infected (suffering from a disease) are not necessarily synonymous. A common analogy is that a single glass of water will not harm you and may even do you good, but the rapid consumption of a thousand glasses of water would kill you. Does that mean that water is beneficial or poisonous? Clearly, it can be both and, likewise, harm is dependent upon the context in which it is being considered. For human parasites, one should also consider the context and psychological consequences. Among some poor communities, being infected by lice and parasitic worms may be considered an unremarkable fact of everyday life. By contrast, in affluent communities, the very thought of harbouring worms inside the body or being bitten by fleas may cause mental torment far above any physical harm caused. It is therefore not a good idea to make the ability to record measurable harm as a pre‐requisite for the classification of the relationship between two organisms. Indeed, in certain instances, low levels of parasitic infection may benefit the well‐being of the host (Maizels 2020). Nevertheless, many parasite species have the capacity to cause morbidity, that is, a diseased state, and some may cause mortality (death). We discuss the possible beneficial consequences of low parasite burdens in more detail in Chapter 12.

The morbidity that parasite infections induce is often reflected in a reduction in the host’s fitness as measured in terms of its growth or reproductive output. This is often attributed to the direct pathogenic effect of the parasite, such as through the loss of blood and the destruction of tissues or competition for resources. For example, many gut helminths act as so‐called kleptoparasites (literally, thieving parasites) and compete with their host for nutrients within the gut lumen. However, the situation is far more complicated than this. Although a functional immune system is crucial for an organism to protect itself against pathogens, immune systems are energetically costly and when nutrients are limiting, it must trade these costs against other physiological processes. Ilmonen et al. (2000) demonstrated this by injecting one group of breeding female pied flycatchers (Ficedula hypoleuca) with a diphtheria‐tetanus vaccine and a control group with a saline solution. The vaccine was not pathogenic and did not induce an infection, but it activated the birds’ immune system. They found that birds injected with the vaccine exhibited a lower feeding effort, invested less in self‐maintenance and had a lower reproductive output, as determined by fledgling quality and number. The authors therefore concluded that the energetic consequences of activating the immune system can be sufficient to reduce the host’s breeding success.