Maverick Virus

The Maverick Virus (イレギュラーウィルス Iregyurā Wirusu, Irregular Virus in Japan) is a computer virus in the Mega Man X series that causes Reploids and Mechaniloids to go Maverick. As its original host, Zero carries the Maverick Virus with him for many years, and he has a unique response to the virus and its variants. After infecting Sigma, the virus merged with his mind to become the Sigma Virus.

Recessional of the Red Dragons[]

The Maverick Virus was indeed still inside Zero, and Sigma decimated Earth after exposure to it, before the new variant of the Sigma Virus. Both variants turned fellow peaceful Reploids into deadly Mavericks, and X and Zero can relate to Spike being corrupt by the neo-Red Dragons. It's possible that like in X4, with dreams of Wily, Zero had a flashback of Iris's death thus with him always comparing Julia to her as he saved her, performed the Dragons' necromancy, deleted the procedure, and always is protective of her.