Parasitism is a biological relationship in which one organism, the parasite, lives on or inside another organism, the host, deriving nourishment from it and generally causing it harm without necessarily killing it. This concept lays the vocabulary foundation for every life cycle studied later in the chapter.
Facultative vs obligate parasites. A facultative parasite can live a completely independent, free existence but will adopt a parasitic habit opportunistically if it enters a suitable host — parasitism is optional for it. An obligate parasite, however, must spend at least one stage of its life cycle in or on a host to survive and reproduce; it cannot complete its life cycle any other way. Entamoeba histolytica, Plasmodium vivax, Ascaris lumbricoides and Wuchereria bancrofti are all obligate parasites.
Host types. A definitive host is where a parasite attains sexual maturity or reproduces sexually (e.g., the mosquito for Plasmodium). An intermediate host is where the parasite's larval stages or asexual multiplication occur (e.g., humans for Plasmodium). A vector, usually an arthropod, actively transmits the parasite between hosts; a reservoir host is an alternative (often animal) host that keeps the parasite circulating in nature and can re-infect humans.
Mechanical vs biological vectors. In a mechanical vector, the pathogen is simply carried passively on the vector's body without any development inside it (e.g., houseflies carrying bacteria on their legs). In a biological vector, the pathogen actually develops or multiplies inside the vector's body before it becomes infective again (e.g., Anopheles mosquitoes for Plasmodium).
Hyperparasites and parasitic castration. A hyperparasite is a parasite that itself parasitises another parasite. Parasitic castration is a phenomenon where a parasite damages or destroys a host's reproductive organs (functionally sterilising it) without killing the host, redirecting the host's reproductive energy toward growth and toward supporting the parasite instead.
Neoplasia, hypertrophy, hyperplasia. Neoplasia is abnormal, uncontrolled cell proliferation forming a useless mass of tissue (a tumour). Hypertrophy is enlargement of individual cells (no increase in cell number); hyperplasia is an increase in the number of cells through division, which also enlarges the tissue.
Health and disease. Health is a state of complete physical, mental and social well-being, not merely an absence of disease. Disease is any deviation from normal bodily structure or function that causes discomfort; diseases are classified as communicable (spread from person to person via pathogens) or non-communicable. This entire chapter's parasitic and infectious diseases fall under the communicable category, and understanding these host-parasite terms is essential to understanding how each of them is transmitted, maintained in a population, and ultimately controlled.