Mostly a parasite, on gaining entry into the body of the host, migrates within the host body in search of a suitable site and finally becomes established in that region of the host body where it lives and reproduces. However, sometimes the parasite happens to reach a place which is not its usual site of localization. Such parasite which fails to reach its normal destination is termed as aberrant.
The parasites are also categorized into two different types on the basis of the number of hosts required for the completion of their life history. The parasites that complete their life cycle in one host only are called as monogenetic. Some parasites display alternation of the host as well as alternation of generation. These parasites which require two different hosts for the completion of their life cycle are called as digenetic. Generally, out of the two hosts one is a vertebrate host which is referred as definitive or final or primary host and the other is invertebrate host, commonly known as secondary or intermediate host.