Viruses

Viruses (sometimes known as germs) act as enemies in the Dr. Mario series. Their maniac design and weird dancing sets them apart from most other villains. Dr. Mario has to link his megavitamins together by color to defeat the viruses before they start spreading a massive flu.

In Smash Bros.
In Super Smash Bros. Melee, Dr. Mario appeared as a playable character. Interestingly, the viruses weren't part of his move set even though they were one of the big parts of the original Dr. Mario. Instead, the viruses appear as a trophy along with Dr. Mario's megavitamins (which are in his moveset).

The viruses also appear in Super Smash Bros. Brawl as stickers. The Blue Virus, Red Virus and Yellow Virus can be obtained at random, but unlike Melee, there is no trophy based on the viruses to be found in Brawl.

Trophy Description
These contagious viruses make life difficult for Dr. Mario. They infect everything they come into contact with, and the good doctor must attempt to neutralize them with his trusty megavitamins. Dr. Mario can stop these pesky germs by lining up the megavitamin capsules of the same color with the nasty microorganism.