An obligate parasite or holoparasite is a parasitic organism that cannot complete its life-cycle without exploiting a suitable host. If an obligate parasite cannot obtain a host it will fail to reproduce. This is opposed to a facultative parasite, which can act as a parasite but does not rely on its host to continue its life-cycle. Obligate parasites have evolved a variety of parasitic strategies to exploit their hosts.
It is advantageous for the parasite to preserve the health of its host when this is compatible with its nutritional and reproductive requirements, except when the death of the host is necessary for transmission.[1]