The MSX platform has never been suffered a lot from viruses. As far as we know, there's only one MSX virus out in the open: the ZAPP virus, for which a nice antivirus utility exists as well.
This Spanish article describes the basic principles of developing a virus for MSX, whilst giving general information on how such a virus works. MSX developers: please use this knowledge wisely. One MSX virus out in the open is more than enough
Relevant link:
How to write a virus for MSX - Tutorial
Just to make things clear, I would not ever give away code that could harm MSX hardware, software or users on purpose.