Z-731 is a virus in the expansion Undead Nightmare that turns the population into Zombies. It only makes an appearance in the non-canon universe. It does not exist in the normal canon.
Zombies are people infected by Z-731, either through bites or direct injection. Their only purpose is to find, infect, and devour humans. Zombies are rather fragile and easier to kill than uninfected humans. Zombies can also be created from normal corpses.
When bitten by a zombie it takes on average around 24 hours to become one, though it varies greatly from person to person, and can sometimes depend on where the person is bit (for example, a person bit in the neck will transform within seconds, while an arm bite may take longer). However, if a person is bitten by more than one zombie multiple times, then this may quicken the process.
The only exceptions are those who have the cure inside of them, including Niko Bellic. This is meant for gameplay purposes, but Alex Vogil does give Niko the cure for the virus before the outbreak begins in Southern San Andreas.
Z-731 was initially created in the 1980s by doctors in Southern America. Starting as a pathogen, it would replicate dead cells and make them live again, essentially cloning the required cells to make a creature live after death. Scientists saw an incredible opportunity and hoped to use it to replicate dead tissue of meat to prevent further slaughter of animals and to repair the damaged tissue of soldiers.
Eventually in 1991, the IAA would catch wind of the experiments being performed with the pathogen and would confiscate samples and using it for their purpose. The samples were modified and it eventually became a virus. Instead of having to be manually inserted into cells, the virus could overwhelm existing cells and take control of them. Fluids such as blood are susceptible to infection.
In 2006 the virus' development would stop after it was not receiving much funding due to it being a matter of "playing god" as Alan Platterson would put it. Spark Incorporated would eventually purchase the rights to the virus' research (and eventually mass production). Alex Vogil orders that the virus be able to pass through different liquids, such as water and urine, to allow easier infection. His plans were ultimately to create the "endgame" bio-weapon in case it would ever be needed.
In 2014, the virus is unleashed onto the population of Southern San Andreas, resurrecting the already dead, eventually infecting the living. The release of the virus is blamed on Niko Bellic.
Zombies are most dangerous when in groups. They tend to stay close to each other because of their reliance on each other for survival.
When attacking an enemy, Zombies are prone to scratching and inevitably biting their victims. The neck is the most common target for their bite.
Confirmed Undead characters
NAME | CAUSE OF INFECTION | LOCATION