Vaccines are consumables necessary for survival if the player is infected. Vaccines will be able to cure 20% the Zombie Virus or get rid of it completely if under 20%. Some Vaccines will be found randomly like other loot but the most effective vaccines can be created from Stem Cells, harvested from the extremely dangerous Stem-Cell carrying zombies.
Now you can get infected by zombies. As such, you need vaccines in order to keep yourself alive. You can see your percentage of blood toxicity as a number set in between the hunger and thirst bars. If the toxicity rises to the one-hundred percent mark you will lose health fairly quickly.