A virus I wrote in the '90s did just that - every time you opened a file, it damaged a small random part of it. My plan was also to randomly change the digits of numbers when printed on paper. I wrote this because a friend of mine managed to steal my source code during my naive XOR encryption. The "encrypted" files had a bunch of zeroes, and he saw my password. My virus worked. First, it damaged his working copy; then he tried to copy the backup, but in doing so, he damaged it, too, even though he claimed his backup diskette was write-protected. The virus didn't spread because I knew how evil it could be - just as evil as MS-DOS, which would run a hidden .COM file without thinking twice. So many fast typists like me were often mistyping "DIR" as "DUR", so I just put my virus in a hidden DUR.COM, copied it into the empty allocated space of COMMAND.COM (less than 200 bytes), and printed out "Bad command or file name". So, the theft was avenged!

P.S. I didn't specify, I didn't have my own computer, only used computers at school and my friend's personal computers, leaving "encrypted" files in case I forgot to bring my diskette. So, after he stole my code and some personal files, her never left me alone on his computer, but he never thought I would infect his computer so easily right in front of his eyes without him realizing it!