A Microsoft-Transmitted Disease. Virus, worm, Trojan horse, whatever you want to call it, an MSTD is unfriendly code which propagates due to Microsoft's insistence on distributing operating systems and applications that resemble Petri dishes. The company's scripting-everywhere policy seems to be the main culprit (scripted email, scripted web browser, scripted word processor). Scripting is on by default, and the security controls which one would expect are often absent or poorly implemented. The best-known examples - so far - are Melissa and ILOVEYOU.