These are very dangerous memory resident parasitic viruses. They trace INT 13h, 21h, then hook INT 21h and write themselves to the end of COM files that are executed. The viruses have a bug and often halt the system while infecting a file. On 4883rd (1313h) infection the viruses erase the MBR of the hard drive. The viruses contain the text string:
MbR killer v0.01
