Computer viruses can not infect write protected disks or infect written documents. Viruses do not infect compressed files, unless the file was infected prior to the compression. [Compressed files are programs or files with its common characters,etc. removed to take up less space on a disk.] Viruses do not infect computer hardware, such as monitors or computer chips; they only infect software.
In addition, Macintosh viruses do not infect DOS / Window computer software and vice versa. For example, the Melissa virus incident of late 1998 and the ILOVEYOU virus of 2000 worked only on Window based machines and could not operate on Macintosh computers.
One further note-> viruses do not necessarily let you know they are present in your machine, even after being destructive. If your computer is not operating properly, it is a good practice to check for viruses with a current "virus checking" program.