from what I understand its based on TWD (The Walking Dead) which means that it's an engineered disease that escaped, and every person on earth is infected and show doesn't deal with it being able to be cured as there doesn't appear to be any scientists left -legasp- so i believe some freedom of expression may be taken with that.
The one that I did a while ago dealt with a cure but it could only be used on those that haven't changed yet, and only worked once.