As described earlier, computer viruses, like their biological counterparts, need a carrier. Biological viruses latch on to animals or insects that transmit them from one host to another. Similarly, computer viruses infect a perfectly normal program and turn it into its `carrier`. In a broad sense, any program, when infected by a virus can be called its `Trojan part` because it now does things which it was never intended to do. Normally, however, the word Trojan is associated with those non replicating programs which are tailor- made for destruction.

Viruses are the next step in the evolution of destructive programs. By definition, it is this ability to replicate that makes them spread far and wide and cause havoc on computers around the world. However, they cannot exist on their own, unlike worms and have to attach themselves to some other code. For the propagation of a virus, a host (Trojan part/carrier) is essential. The host could be the boot sector, the partition table or an executable file. When the infected code gets executed, the viral code enters the memory and performs what it has been programmed to do.