Zombie Virus enables Romero to produce a pinkish gas that, when inhaled, transforms victims into zombies. Those infected gain the endurance of a zombie, increased strength and the ability to pass along the virus via bite. If a hero is infected by the gas, they can still use their Quirk and retain aspects of their personality. They also moan and have blueish-grey skin.
The only weakness of this Quirk is the user is not immune to it. An example of this is when Romero is bitten by an infected Katsuki Bakugo, resulting in Romero also turning into a zombie. It's unknown if it’s possible to reverse the Quirk if the user is not a zombie or before time runs out.
Also, anyone who has a zombie-like appearance will be seen to the zombies as one of their own. An example of this is when Toshinori Yagi deactivates One For All, the zombie heroes leave him be, believing that he is one of them.