These are harmless memory resident parasitic viruses. They hook INT 21h and write themselves to the end of .COM files. The viruses infect only files that are archived to PKZip files - when .ZIP file is opened, the viruses start to intercept .COM files opening, infect them and disinfect on closing. As a result the files on disk are not infected, but in ZIP archives there are their infected copies.
The viruses do not manifest themselves in any way, they contain the text:
TANxxxxxxx
