[The importance of environmental factors. 1. Conditions for parasite infection]

Results of research showing an evident influence of the host's species, strain and age on the biology of the parasite have been presented. Various Hymenolepididae and Amidostomum anseris served as models. Irregular growth and development of the parasite in a given host species cannot be used as a sure criterion to determine the quality of the host-parasite system, because this irregularity occurs sometimes in a specific host, too. It is proposed that a host can only be regarded as specific for a particular species of parasite if its life cycle can be completed and development of successive generations can be assured. Not only the main host but an auxiliary, incidental or side one can play the role of a specific host.