[1] Obligate parasitism is exhibited in a range of organisms, with examples in viruses, bacteria, fungi, plants, and animals.
[7] Due to the wide range of obligate parasite types, it is impossible to identify a general invasion strategy.
Intracellular parasites use various strategies to invade cells and subvert cellular signalling pathways.
[10] One of the mechanisms that hosts employ in their attempt to reduce the replication and spread of pathogens is apoptosis (programmed cell death).
Some obligate parasites have developed ways to suppress this phenomenon, for example Toxoplasma gondii, although the mechanism is not yet fully understood.
[13] However, there is a difficulty in demonstrating changes in behaviour are the result of a selective process favouring transmission of the parasite.
[15] Most behaviour changes have not been demonstrated to lead to fitness gains in either the host or the parasite.
[16] An example of this behaviour is the attraction of rats to cat urine after infection with Toxoplasma gondii.
[13] Three main evolutionary routes have been suggested for the appearance of host behaviour manipulation by parasites.
[20] The cowbird and cuckoo require the nests and parental care of other passerines in order for their young to fledge.
An example is Niphanda fusca, a butterfly that will release cuticular hydrocarbons (CHCs) to trick the host ant, C. japonicus, into adopting the larva as their own in their own nest.
[29] Current theory in evolutionary biology indicates that host-parasite relationships may evolve towards equilibrial states of severe disease.
[30] This differs from the conventional belief that commensalism is the ideal equilibrium for both the host and parasite.