The paper describes the participation of parasites in the regulation of the physiology of development and reproduction of their hosts. The observed increase in the hosts' lifespan is shown to confer a significant selective advantage for a pathogen in some parasitic systems. The parasite-induced increase in the longevity of a host is accompanied (provided) by its reduced reproduction. The pointed parasite-induced increase in the life of an infected host may be caused by parasite genes in the context of the Dawkin's expended phenotype theory.