Previously, in Chapter 9, we defined a parasite as an organism that obtains its nutrients from one or a very few host individuals, normally causing harm but not causing death immediately. We must follow this now with some more definitions, since there are a number of related terms that are often misused, and it is important not to do so.
When parasites colonize a host, that host is said to harbor an infection. Only if that infection gives rise to symptoms that are clearly harmful to the host should the host be said to have a disease. With many parasites, there is a presumption that the host can be harmed, but no specific symptoms have as yet been identified, and hence there is no disease. 'Pathogen' is a term that may be applied to any parasite that gives rise to a disease (i.e. is 'pathogenic'). Thus, measles and tuberculosis are infectious diseases (combinations of symptoms resulting from infections). Measles is the result of a measles virus infection; tuberculosis is the result of a bacterial (Mycobacterium tuberculosis) infection. The measles virus and M. tuberculosis are pathogens. But measles is not a pathogen, and there is no such thing as a tuberculosis infection.
Parasites are an important group of organisms in the most direct sense. Millions of people are killed each year by various types of infection, and many millions more are debilitated or deformed (250 million cases of elephantiasis at present, over 200 million cases of bilharzia, and the list goes on). When the effects of parasites on domesticated animals and crops are added to this, the cost in terms of human misery and economic loss becomes immense. Of course, humans make things easy for the parasites by living in dense and aggregated populations and forcing their domesticated animals and crops to do the same. One of the key questions we will address in this chapter is: 'to what extent are animals and plant populations in general affected by parasitism and disease?'
Parasites are also important numerically. An organism in a natural environment that does not harbor several species of parasite is a rarity. Moreover, many parasites and pathogens are host-specific or at least have a limited range of hosts. Thus, the conclusion seems unavoidable that more than 50% of the species on the earth, and many more than 50% of individuals, are parasites.
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