These are dangerous memory resident parasitic viruses. They hook INT 21h and writes itself to the end of COM- and EXE-files.
Angel_2.661
It hits the files that are executed. The virus checks the code of the programs that are executing, and in some cases patches that code (to disable some anti-virus scanner?). The virus contains the internal text string:
v6 Angel
Angel_2.1571
It is an encrypted virus. It hits the files that are accessed. The virus has the bug and can infect data files as COM ones.
