They may be spread by multiple users on one microcomputer or by copying a file from an electronic bulletin board or downloading a file from a mainframe. They may be spread by simply moving a data file from one microcomputer to another microcomputer. In college microcomputer labs, viruses are spread by the use of the computer by several users during a day. As a prank, a person may introduce a virus to a DOS disk that is used on several microcomputers during the day. Another way a virus can spread is by employees taking floppy disks home and working on them on their home computers.
Microcomputers are especially vulnerable.
The widely established standards of microcomputer software make viruses easier to distribute. The introduction of a virus into a single microcomputer in a network can result in the infection of the entire system. There is microcomputer virus screening software on the market that promises to protect computer systems against infection. This software impairs any writing to a protected file. Any preventive software can only protect against known viruses.