Definitive host: The definitive host is the one in which a parasite reaches sexual maturity and undergoes reproduction. It is mostly a vertebrate.
Intermediate host: The host in which some development of the parasite occurs but it does not reach sexual maturity, is referred as intermediate host and is usually a invertebrate.
Paratenic or transport host: Sometimes the parasite enters a host in which it does not undergo any development but remains alive till it gains entry in the definitve host or intermediate host. Such a host is termed as paratenic or transport host or a carrier host. These hosts are important for the completion of the life cycle of certain parasites as they are believed to bridge the ecological gap between the intermediate and the definitive
Reservoir host: An animal which is utilized by a parasite as a temporary refuge till it reaches its appropriate host is referred as reservoir host. It is generally a animal which is normally infected with a parasite that can also infect man. For example, dogs and cats are reservoirs of Leishmania.
All the parasites follow one principle as far as the selection of the host is concerned. Some parasites are specialized to a single species of the host where they live and multiply, while others have become adapted to a wide variety of hosts. For example, a malarial parasite is found in man as well as in birds but the species of malarial parasite found inman is not the same as the one found in the birds. This is called as host- specificity.
Another notable feature of the parasite is that, the parasite can infect different hosts but within each host it can survive in definite tissues or organs only. This can be explained by the example of malarial parasite which is found in different hosts like man and birds but in either of these hosts the tissue which it has selected is same i.e. blood. The parasites are, thus, not only host- specific but tissue- specific also.
The parasite causes damage to the host and for every parasite to act as a disease agent there has to be its source or reservoir. It must also adopt a definite system to gain entry and thrive well in the body of the host in order to derive maximum benefit from the host.
The parasite can only be called successful if it gets all the advantages from the host without endangering life of the host because the parasite itself would have no future if the host dies. The manner in which the parasite makes its way in the body of the host and the extent to which it can cause damage is often studied under the heading of dynamics of disease transmission.