If Melissa can infect 1.2 million computers in the hours before a fix is released, that's a lot of damage. Even a few minutes' thought yields some pretty scary possibilities. Sure, it's unwise to run executables from strangers, but both Melissa and Worm. ExploreZip arrive pretending to be friends and associates of the recipient. ExploreZip even replied to real subject lines.
Simple patches are inadequate.
A large distributed system communicating at the speed of light is going to have to accept the reality of infections at the speed of light. Unless security is designed into the system from the bottom up, we're constantly going to be swimming against a strong tide.