A virus that damages the item it has infected by destroying one or more bytes is not removable. Replication Viruses replicate by nature. A well-designed virus will replicate for a long time before it activates, which allows it plenty of time to spread. Resident Loads much like a TSR staying in memory where it can easily replicate itself into programs of boot sectors. All boot viruses are resident viruses, as are the most common file viruses.
Macro viruses are non-resident viruses.
The purpose is to fool people who use integrity checkers into thinking that modifications reported by their integrity checker are due solely to legitimate reasons. By infecting less often, such viruses try to minimize the probability of being discovered. The ability to hide from detection and repair manifests in two ways. 1.Full - Virus redirects disk reads to avoid detection. 2.Size - Disk directory data is altered to hide the additional bytes of the virus.