Viruses

Viruses are the enemies of the Mega Man Battle Network series. They start as corrupted data in electronic devices. After that, the data becomes a virus and proceeds to disrupt the operating system or cyber network of the device. Viruses are also found randomly across the Net, searching for NetNavis and programs to delete. They may leave chip data upon deletion (the chip depending on the virus, and related to the attack that the virus uses).

As of Battle Network 6, there are about 112 different species of virii, not including the AlphaBug line.

Uses
While viruses are often wild, some are man-made. They can be created to guard important data or switches. Villains often use viruses to trap data in an attempt to delete (or at least stall) Navis that may be trying to stop them. They are also commonly used by villains to shut down machinery.

Some Navis are capable of calling viruses for use in battle. A good example is MagicMan in the first Mega Man Battle Network, but also HeelNavis commonly use viruses to attack their opponents (although they often use viruses to do all of their fighting, some that fight in battle themselves are capable of calling viruses mid-battle).

In Mega Man Battle Network 3, MegaMan is capable of calling a virus to launch an attack against an enemy. Rush is also capable of calling viruses in battle.

In the anime, some Battle Chips can summon a virus to execute an attack. For exampke: Condor summons a Fishy3 to ride on, IceBall summons a ColdHead that fires a frozen cannonball, and Trident summons a Piranha to fire a trident spear.