These are not dangerous parasitic and companion viruses. During execution they search for .EXE-files and create companion .COM-files. Then they search for .COM-files (including COMMAND.COM) and write themselves at their ends. As the result the companion files are infected twice.
"Bug.920" is not memory resident virus, "Bug.1497" hooks INT 21h and infects files that are executed.
The viruses delete CHKLIST file. Depending on the system date they change the font of a random selected letter to "bug" image. The viruses contain the internal text strings:
*.com *.exe c:chklist.* c:command.com
