The Structure of a Virus

Every viable computer virus must have at least two basic parts, or subroutines, if it is even to be called a virus.

Firstly, it must contain a search routine, which locates new files or new disks which are worthwhile targets for infection. This routine will determine how well the virus reproduces, e.g., whether it does so quicklyor slowly, whether it can infect multiple disks or a single disk, and whether it can infect every portion of a disk or just certain specific areas. As with all programs, there is a size versus functionality tradeoff here. The more sophisticated the search routine is, the more space it will take up. So although an efficient search routine may help a virus to spread faster, it will make the virus bigger.

Secondly, every computer virus must contain a routine to copy itself into the program which the search routine locates. The copy routine will only be sophisticated enough to do its job without getting caught. The smaller it is, the better. How small it can be will depend on how complex a virus it must copy, and what the target is. For example, a virus which infects only COM files can get by with a much smaller copy routine than a virus which infects EXE files. This is because the EXE file structure is much more complex, so the virus must do more to attach itself to an EXE file. In addition to search and copy mechanisms, computer viruses often contain anti-detection routines, or anti-anti-virus routines. These range in complexity from something that merely keeps the date on a file the same when a virus infects it, to complex routines that camouflage viruses and trick specific anti-virus programs into believing they’re not there, or routines which turn the anti-virus they attack into a logic bomb itself.