The term “sparse infector” is sometimes used to describe a virus that infects only occasionally (e.g. every tenth program executed), or only
files whose lengths fall within a narrow range, etc. This is a type of virus that only infects files when certain conditions are met.It is logic dependent virus. Sparse is a Memory Resident Infector, File Infector virus of .COM files including COMMAND.COM.
The virus that affect the operating system are system infector. The “system infector” viruses infect the various parts of the operating system or master program software. This virus prevents the computer from booting.The virus prevents the computer from booting.It infects the .EXE,.COM or .SYM files.It is also called file infector virus.
System infectors differ from boot infectors that system infectors gain control after the computer is booted and infects a hard disk or bootable floppies which contain system files only.