These are not dangerous memory resident encrypted parasitic viruses, "X-Mrei.1405" is the polymorphic virus. They are encrypted in the system memory as well as in files. While installing memory resident the viruses trace INT 21h chain and patch DOS kernel to hooks INT 21h calls. The viruses then write themselves to the end of COM and EXE files that are executed or opened. They check file names and do not infect anti-viruses AIDSTEST, DRWEB, SCAN as well as WIN.COM.
On each INT 21h call the viruses display text "X-Mrei 7" to the 26th line on the screen. This line is out of standard screen size and is visible only is current video mode supports more than 25 lines one the screen.
