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. Other Virus Categories Viruses, whether they are boot viruses, file viruses, or macro viruses, can employ none, one, or several of the following techniques to spread or conceal themselves. Multi-partite viruses often infect multiple targets instead of just one type of file or disk. 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. These viruses actively conceal themselves while they're running in memory. If the anti-virus program doesn't scan in memory for these viruses, it will completely miss them when scanning files. These viruses are designed to actively attack anti-virus software. Viruses don't infect compressed files.