Parasites may possess single or a wide range of hosts in their life cycle. They may be:
Monoxenous i.e if they possess a single host,
Oligoxenous i.e if they utilize a small range of host
Polyxenous, i.e if they use many suitable hosts.
Parasites with indirect life cycles are less host specific than those with direct life cycles. Thus there is less host specificity with increase in the number of intermediate host. Co-evolutionary factors are important in parasites that feed on host tissues or occur in extra intestinal sites. This is because the parasite must present the right cues/signals and respond appropriately to host defense system. The inability of a parasite to respond to such cues may act as a barrier on its host range.
The host range or host specificity of a parasite is the collection of hosts that an organism can utilize as a partner. In the case of human parasites, the host range influences the epidemiology of the parasitism or disease. Specificity in parasites is seldom absolute. It differs amongst various types of parasites and at different stages of the parasite life in the definitive and intermediate hosts. Majority of parasites however are restricted to just one or few closely related species.
Supra –specific condition occurs when groups of parsites are associated with natural groups of host.
Infra-specific conditions on the other hand occur when a single species of parasite is associated with a single host. Over 70% of described monogenetic trematodes are infra- specific.
Phylogenetic, physiological, ecological and human factors affect parasite specificity. Specificity is the effect of biological compatibilities between hosts and parasites. The range of host can be limited by lack of host stimuli, host immune response, innate resistance and simply the availability of essential nutrients to the parasite.
Localization of, establishment, growth and reproduction are factors which contribute to host specificity. Behavioral adaptations have also contributed tremendously to increase the chances of host location. Chemical cues emitted by hosts play vital roles in the infective stages locating or finding their hosts. Host selection is determined by morphological and physico-chemical factors.t