The 1985 cult classic Return of the Living Dead is remembered best for giving us the first zombies ever to say “brains”, and the first to eat brains, but it also presented the hypothesis that ash from a cremated zombie could be highly infectious. What if ROTLD was on to something, and the source of the undead plague can’t be destroyed by fire? What if the sickness spreads even faster once ignited?
If we take as fact that the zombie pathogen is passed from one person to another through bodily fluids (bites, blood, etc), then the sickness must exist on the microbial level. Microbes are microscopic entities that live in the water you drink, the food you eat, and the air you breathe. Most microbes are helpful, but others have proven to be killers on a massive scale, including smallpox, flu, tuberculosis, malaria, plague, measles, and cholera.
There are some microbes that can survive fire. Extremophilic microbes actually thrive in extreme conditions that would kill other life forms. If the zombie microbe can survive in a human corpse long after death, there is no reason to think it couldn’t also survive a little heat.
Though it’s quite possible that the infectious agent in zombies is burned up right along with the body, it also might be possible that all or part of it would survive the fire. An inhaled particle, or an inadvertent rub of your eye, and you could be doomed to suffer a slow sickness, death, and reanimation without ever having come in direct contact with a zombie.
So the next time you find yourself in a catastrophic zombie outbreak and a member of your group wants to torch a shambling corpse or two, you might want to think twice about handing over the gas can. If nothing else, you’ll prevent a flaming zombie from running around the neighborhood lighting everything else on fire.