AIDA or Artificially Intelligent Data Anomaly, are system abnormalities that appeared in The World R:2 after the disappearance of Aura. Once appearing in The World, the AIDA mutated into Tri-Edge and infected Ovan. After the infection of Tri-Edge, the AIDA got more aggressive and started to lash out against the players. Some of the AIDA would infect players, creating AIDA-PCs, then PK other players. The players PKed by AIDA-PCs became Lost Ones, who then would be rendered comatose in the real world. Most of the players who where infected in .Hack//G.U. where infected for certain reasons, for example the acquisition of an avatar, so that it can reach its main goal the real world. The only ones that can see AIDA are the eight Epitaph Users and other AIDA-PCs.
Though the AIDA seen in .Hack//G.U. is aggressive, there is different types of AIDA which can be seen in .Hack//ROOTS Returner. This AIDA was only like this because all it saw was the hate and anger of humans.