These are dangerous memory resident parasitic viruses. They hook INT 21h, 28h and infect COM files that are executed. The viruses have bugs and halt the system if there is no UMB memory. While infecting they write themselves to the end of the file. The viruses do not infect files immediately when they are executed, but delay it up to INT 28h call (DOS internal idle). So they infect files in the "background".
"Andy.998" also hooks INT 13h and on 15th of any month writes data to disk instead of reading. This definitely corrupts data on the disk. "Andy.1016.b" hooks INT 13h as well, but it disables writing to disks on any day, that corrupts data that is copied or modified. "Andy.1016.a" hooks INT 1Ch and depending on their internal counters changes color of the screen and disables keyboard.
The viruses contain the texts:
"Andy.998": ANDY-3
"Andy.1016.a": ANDY-1
"Andy.1016.b": ANDY-2
