The classical definition states that a parasite is an organism that, in some phase of its life cycle, utilizes the organisms of some other host as a source of food and as a permanent or temporary environment for its life, and thus harms it either directly or indirectly. Thus parasites are not defined taxonomically but ecologically and include organisms from viruses through tapeworms to the Amur bitterling (Rhodeus sericeus). Like most biological definitions, even this one is not capable of describing the facts in their full complexity. In a number of cases, especially for most phytoparasites, we are not able to exactly differentiate between a parasite and a predator. In addition, problems are encountered with nidicolous parasites, i.e. parasites that do not live in or on the bodies of their hosts, but in their dwellings. From the viewpoint of the negative impact on the host organism, there are a wide range of parasites, from species that are almost innocuous (in this case, we speak of commensals and not parasites) to species that almost always kill their hosts – parasitoids. Parasites differ from predators and micropredators (including, for example, mosquitoes) in that the hosts provide a permanent or temporary environment for their lives. This difference is of fundamental importance from the standpoint of evolution of parasitic species. While the relationships between a predator and its prey (as two individuals) are only antagonistic, the parasite – host relationship is, to a certain degree, asymmetric. The host has quite the opposite interests to those of the parasite In contrast, a parasite, in that it generally requires a live host as its environment for life, has at least a certain interest identical with that of its host – a host attacked by a parasite must live for at least some time and, in some cases, where possible, also reproduce, for example in the case of the possibility of transovarial transmission of the parasite from parents to progeny. This fact, together with the necessity of developing mechanisms capable of overcoming systems of specific physiological (immune) defense of the host requires maximal evolutionary adaptation of the parasite to the host. The host organism reacts evolutionarily in some way to this adaptation and develops the relevant counter measures, so that the evolution of a parasite tends to have the character of co-evolution of the parasite – host pair.