These are very dangerous memory resident parasitic viruses, "YZ.1339" is an encrypted virus. They hook INT 21h ("YZ.1230" also hooks INT 1Ch) and write themselves to the end of COM and EXE files that are executed. On March 8th they erase the disk sectors and display the message:
It is my birthday!!!
"YZ.1230" does that at 15:00, "YZ.1339,1434" - while infecting a file. "YZ.1339,1434" also beeps by PC speaker.
The viruses contain the ID word:
YZ
