These are dangerous nonmemory resident parasitic viruses. They search for
.COM files of the current directory, then write themselves to the end of
the file, and Jmp-Virus instructions (four bytes: XCHG AX,DX; JMP
Loc_Virus) to the file header. Depending on the system timer these viruses
might reboot the computer. They contain the string “.COM”. On an attempt to
infect they open the files and might left them opened.