Though some virus's do little but replicate others can cause serious damage or effect program and system performance. A virus should never be assumed harmless and left on a system. Boot Record, Master Boot, FAT and Partition Table. A virus is inactive until the infected program is run or boot record is read. As the virus is activated it loads into the computers memory where it can perform a triggered event or spread itself.
Disks used in an infected system can then carry the virus to another machine.
Programs downloaded from bulletin boards can also spread a virus. Data files, however, can not transfer a virus but they can become damaged. Once the boot code on the drive is infected the virus will be loaded into memory on every startup. From memory the boot virus can travel to every disk that is read and the infection spreads. Most Boot virus's could be on a system for a long time without causing problems.