Zombie

Zombies are main antagonists of the series.

Cause
The cause of the virus that zombifies people is unknown. It is theorized that it was likely the result of genetic and bioengineering for the military, as said by Tsuru.

Infection
The virus can be spread if a zombified individual bites a living being or if a living being consume flesh of a zombified being. Once it happens it takes some minutes to hours to happen varying on the person with Shizuka taking longer to get infected than most.

Physiology
When infected their veins and eyes are exposed. Sometimes mutations can happen like the Chairman getting larger or Zombified Shark spouting out human legs it ate from earlier to walk on land.

However, Zombies can’t actually see well and mostly rely on sound as they attracted to loud sounds which can be dangerous for those who panic easily.

Some Zombies also maintain skills and abilities they had when they were alive. Like the sprinter zombies from the Aquarium they were able to run at great speeds and not loose any momentum whilst chasing down humans

Protection and curing
If someone wears protection that stops biting they won’t be infected like Akira’s superhero suit made from a shark diving suit though it will still hurt from the pressure of the bite. Izuna has complete immunity to Zombie Virus as her immune system will destroy the zombie virus thus she will never become a zombie.

While there is no way to turn a zombie back to normal so far, there has been a serum made to help protect against it, such as one used for Shizuka when she got bitten by a zombie using Izuna’s blood who has immunity so that it stopped the virus from infecting her.