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Considerable confusion surrounds the terms pathogen and parasite. While they are generally used to describe microbial disease agents, in particular the fungi, bacteria, and viruses, the distinction between the two terms has often been overlooked. They are not synonymous; a parasite is an organism having a particular type of nutritional relationship with a host, while the term pathogen refers to the ability of an organism to cause disease. They may be defined as follows.

 Parasite: an organism or virus living in intimate association with another living organism (host) from which it derives some or all of its nutrients, while conferring no benefit in return.

 Pathogen: an organism or virus able to cause disease in a particular host.

The allied term pathogenesis describes the complete process of disease development in the host, from initial infection to production of symptoms.

At first sight, the distinction between a parasite and a pathogen might appear subtle; indeed, in many cases the parasitic activities of an organism automatically lead to it being a pathogen as well. The diversion of nutrients from the host will cause some metabolic stress which will normally be expressed as disease. However, in other host–microorganism associations this stress may be offset by the microbe contributing nutrients in return. This is the case with root nodules of legumes, where the bacterium Rhizobium obtains carbohydrates from the host but also fixes atmospheric nitrogen, some of which the host subsequently utilizes. Mycorrhizal fungi infect plant roots but actually stimulate growth by assisting the uptake of scarce nutrients, especially phosphates, from the soil. The definition of a parasite given above takes account of situations such as these.

Where the invading microbe confers some beneficial effect, the term symbiosis has been used. As originally conceived, symbiosis (literally = living together) referred to any intimate or close association between organisms, irrespective of benefit or harm, and was subdivided as shown in Figure 2.1.

The advantage of this scheme is that it can accommodate relationships where the balance may shift from mutual benefit, termed mutualism, to injurious effects on one partner.

If one considers the terms parasite and pathogen from the reverse viewpoint, in other words the ability to cause disease, the difference becomes more obvious. While all parasites are potentially pathogenic due to their diversion of host nutrients, many of the characteristic symptoms of disease cannot be explained on the basis of nutritional stress alone. The growth and development of a pathogen in its host, along with the response of the host to the presence of an alien organism, involve other interactions which have little to do with nutrition. Many of the more injurious effects of pathogens may be traced to toxic chemicals whose production may be incidental to their parasitic way of life (see Chapter 8). Looked at in this way, the statement “a good parasite is a poor pathogen” may appear to be justified. Any organism which is dependent upon another organism for its supply of nutrients might be expected to restrict its pathogenic effects to a minimum.

Figure 2.1 Symbiotic relationships: + positive effects on partner; − negative effects on partner.