Cows are<unk> s of African trypanosomiasis In infectious disease ecology and epidemiology, a<unk> , also known as a disease<unk>  or a<unk>  of infection, is the population of organisms or the specific environment in which an infectious pathogen<unk> ly lives and reproduces, or upon which the pathogen primarily depends for its survival. A<unk>  is usually a living host of a certain species, such as an animal or a plant, inside of which a pathogen survives, often (though not always) without causing disease for the<unk>  itself. By some definitions a<unk>  may also be an environment external to an organism, such as a volume of contaminated air or water. Because of the enormous variety of infectious microorganisms capable of causing disease, precise definitions for what constitutes a<unk>  are numerous, various, and often conflicting. The<unk>  concept applies only for pathogens capable of infecting more than one host population and only with respect to a defined target population – the population of organisms in which the pathogen causes disease. The<unk>  is any population of organisms (or any environment) which harbors the pathogen and transmits it to the target population.<unk> s may comprise one or more different species, may be the same or a different species as the target, and, in the broadest sense, may include vector species, which are otherwise distinct from<unk> s. Significantly, species considered<unk> s for a given pathogen may not experience symptoms of disease when infected by the pathogen. Identifying the<unk> s of infectious pathogens has proven useful in treating and preventing large outbreaks of disease in humans and domestic animals, especially those diseases for which no vaccine exists. In principle, zoonotic diseases can be controlled by isolating or destroying the pathogen's<unk> s of infection. The mass culling of animals confirmed or suspected as<unk> s for human pathogens, such as birds that harbor avian influenza, has been effective at containing possible epidemics in many parts of the world; for other pathogens, such as the ebolaviruses, the identity of the presumed<unk>  remains obscure.
Natural reservoir
Disease vector
Louse-feeder
Pest house
Infected blood scandal 
Mosquito
Airborne transmission
Human parasite