Sparse infector-logic dependence viruses

This is a type of Computer virus that only infects files when certain conditions are met.

Sparse infectors use distinct strategies to reduce their detection. They are viruses that infect “sometime”. For example, they will affect a program every tenth performance. Because they are random infectors, antivirus software has a difficult to detect them.

Sparse is a Memory Resident Infector, File Infector virus of .COM files including COMMAND.COM

Strategy for Virus creation

In order to spread widely, a virus must attempt to avoid detection. To minimize the probability of its being discovered a virus could use any number of different techniques. It might, for example, only infect every 20th time a file is executed; it might only infect files whose lengths are within narrowly defined ranges or whose names begin with letters in a certain range of the alphabet. There are many other possibilities.

A virus which uses such techniques is termed a sparse infector

Sparse infector virus infects only occasionally.They will not spread as quickly but will also probably not be discovered as easily.