How do computer viruses spread?

When executing program code that is infected by a virus, the virus code will also run and try to infect other programs, either on the same computer or on other computers connected to it over a network. And the newly infected programs will try to infect yet more programs. When sharing a copy of an infected file with other computer users, running the file will infect the computer and files from that computer may spread the infection to yet more computers.

If a computer is infected with a boot sector virus, the virus tries to write copies of itself to the system areas of floppy disks and hard disk. Then the infected floppy disks may infect other computers that boot from them and the virus copy on the hard disk will try to infect still more floppies. Some viruses, known as 'multipartite' viruses, can spread both by infecting files and by infecting the boot areas of floppy disks.