Virus

Viruses are enemies that mainly appear in the Dr. Mario games. The Viruses main colors are Red, Blue, and Yellow and they can be only defeated by matching up three of the same colors.

Mario & Luigi series
Viruses are first seen in Mario & Luigi: Superstar Saga where they are only faught in Woohoo Hooniversity. In battle, they mainly attack by trying to bite either Mario or Luigi. If this attack isn't blocked back, the Virus may poison the brother.

Viruses are seen briefly in Mario & Luigi: Bowser's Inside Story where they are one of the Toothy's attacks.

Viruses are fairly common in Mario & Luigi: Dream Team where they are usually seen in the areas when Mario and Luigi are searching for the five pieces of the Ultibed. The Viruses always fight in groups and they attack mainly by charging at Mario and Luigi. It should be noted that the Viruses are the only enemies that are faught in a horde that are not faught in the dream world.