The Viruses are anthropomorphic embodiments of disease (albeit in a family-friendly manner) and the main antagonists of the Dr. Mario series as well as the spin-off, Dr. Luigi - like many Mario villains they are comical in appearance and nature but are theoretically very dangerous opponents since they would presumably spread illness across the Nintendo universe if Mario (and later Luigi) didn't keep them at bay.
Due to Dr. Mario being a puzzle-based, simplistic, game the Viruses do not speak nor have much influence but they serve as enemies along the line of famous video-game enemies such as Space Invaders or (fittingly) the original Koopa Troopas, having little backstory but being set as enemies for the hero to defeat.
Much like real-world virsuses the only way to defeat these microscopic creatures was with advanced medicine, symbolised in-game by stacking colored pills ontop of one another to destroy the virus that has the same color as the pills around it.