There are several types of hosts for disease agents. Understanding the different types is important for understanding disease biology and epidemiology. A reservoir host (usually simply called ‘ reservoir ’ ) is an animal that harbors a disease agent. Reservoir hosts must be able to support the disease in the absence of other species, providing a means of long - term persistence. The reservoir host must be able to provide the disease agent to other species, allowing spread of the disease. The reservoir host usually is not seriously affected by the disease, though there may be signs of infection. Primary or definitive hosts are species in which the disease agent passes the adult, sexual, or multiplicative stage of the life cycle. The primary hosts can also be the reservoir hosts. Intermediate hosts are animal species that the disease agent passes through during the immature or nonsexual phase of the disease life cycle. Amplifier hosts are animals in which the disease agent abundance is increased without severely affecting the host. Incidental , aberrant , or unnatural hosts are not the normal hosts for a disease, and although in some cases they are not susceptible, in other cases they are extremely susceptible, displaying overt signs of infection. These are also called dead - end hosts or dilution hosts because they may perish or not support high concentrations of the disease agent, proving to be unsuitable for uptake of the disease agent by a vector.
Diseases normally have an environment or habitat where they persist in a relatively stable way. Here the environment (or host) and the disease agent are co - evolved, allowing both to co - exist and neither to eliminate the other. The modifi cation of habitat, or the movement of new wildlife, livestock or humans into an environment where a disease has evolved a stable relationship with its host can upset the balance and allow establishment of new disease - host relationships. In such situations, wildlife, livestock and humans can prove to be various types of host, and arthropods can prove to be vectors and/or intermediate hosts.
