Peremeesorganismis võib üheaegselt parasiteerida mitmeid eri liike parasiite. Parasiidid mõjutavad sel juhul ka üksteist, vastastikune toime võib olla erineva loomuga, tugevdades või ka nõrgendades üldist patogeenset mõju peremehele.
Actions on lower animals, plants and micro-organisms are not eligible, except for those which are reared or cultivated on land or in fresh water or such organisms which are suited for use as biological control agents in agriculture. Exception will be made in the specific case of defined gene-for-gene relationships between parasite or symbiont and host, and where both organisms are to be conserved.
Parasites live in or on a single host throughout either a stage in their lives or their entire life span, thereby decreasing the survival or reproduction of their hosts.
