It also contains a small program called the boot program that loads operating system files. Boot sector viruses infect the boot program of the hard drive when an infected diskette is left in a floppy drive and the system is rebooted. When the computer reads and executes the boot sector program, the boot sector virus goes into memory and infects the hard drive. Later, when the user boots from the hard drive, the virus again gains control and can then infect each and every diskette used on the computer. For example, they will infect both files and boot records on hard disks or both files and boot sectors on floppy disks.
Polymorphic viruses mutate to escape detection by anti-virus software.
Both polymorphic file, boot sector, and macro viruses have been identified. If the anti-virus program doesn't scan in memory for these viruses, it will completely miss them when scanning files. Viruses don't infect compressed files. However, applications within a compressed file could have been infected before they were compressed. Some viruses are known to insert copies of themselves in already-created archives.