The Most Destructive Viruses of All Time

Hackers and security vendors are engaged in a never-ending game of leapfrog, with the former constantly devising clever viruses and the latter feverishly trying to concoct antidotes. As a result, through the years hundreds of thousands of malignant programs have infected networks around the world. Most have had minimal impact, but a handful have caused hundreds of millions -- even billions -- of dollars in damage.
Some might mistakenly think viruses are a recent problem, but they have been around for at least 20 years. "Viruses have been around since the early 1980s, and their basic design hasn't changed much since then," noted Fred Cohen, a principal analyst with the Burton Group.

In 1980s, one of the first viruses spread by infecting IBM (NYSE: IBM) mainframe systems. But the attack did not gain much notice because it did not infect many systems. At the time, mainframes were found only in large corporations, and many had appropriate security checks in place.
The Michaelangelo virus, which sprouted at about the same time, was the first widespread attack centering on PCs. It made its way around the world but did so very slowly because it was transported mainly when users loaded infected floppy disks on uninfected systems.