However there are some nasty ones that will destroy the boot information or force a complete format of the hard drive. While the virus is in memory any program file subsequently run becomes infected. Multiple infections are very common and will certainly cause system problems. Program files may function without any problems for some time but eventualy programs have problems or multiple infection brings the sytem down. The data the program produces may be a first sign of infection such as saving files without proper DOS names.
Viruses normally have multiple characteristics.
Loads much like a TSR staying in memory where it can easily replicate itself into programs of boot sectors. Not as common. The ability to hide from detection and repair manifests in two ways. Full - Virus redirects disk reads to avoid detection. Size - Disk directory data is altered to hide the additional bytes of the virus.