- "It's a bug from a computer virus... it has an HP that is, infinite."
- — Balmung —
A Data Bug (ウィルスバグ, Uirusu Bagu, lit. "Virus Bug") is an error in The World that affects monsters. Generally it causes the monster to have an infinite Hit Point level, rendering it virtually indestructible. Only through the use of data rewriting programs such as Data Drain can the monster be turned normal again, and defeated.
Data bugs first appeared in SIGN, where rumours of undefeatable monsters began circulating on the BBS. Sora, Crim, and Silver Knight all fought against one at some point or another during the series, and Balmung's only appearance in the anime involved him fighting against a Data Bug.
In the Game series, data bugs were a fairly common enemy. Due to the effects of Kite's bracelet, the data bugs could be defeated, usually leaving behind rare Virus Cores. It was also revealed during the games that the Data Bugs were a result of errors originating from the Morganna program. It was also shown that almost all the monsters in The World were infected with the data bug virus, even if they didn't show any symptoms of them. It was due to this reason that Kite could drain Virus Cores from even normal looking creatures.
In Unison, it is revealed that Data Bugs have mostly vanished from The World, due to the defeat of the Morganna program at the hands of Kite and the .hackers. The few that still remain are being hunted by Kite along with the Descendants of Fianna.
Modified monsters similar to data bugs appear in the Legend of the Twilight anime, created by several hackers, as well as the AI called Morti. Like Kite, Shugo is forced to use his own bracelet to defeat the monsters.
- Data Bugs were called Virus Bugs in the original Japanese version. The reason for this change is unknown.