Viruses, fungi, bacteria and other microscopic invaders have plagued humankind since the beginning of time, shortening our lifespans and often make our lives miserable. Yet it wasn't until the 1800s that we finally figured out that the tiniest of invaders, such as smallpox or HIV, are often the most dangerous of our biological enemies.
Our immune systems, brimming with the weaponry of white blood cells, lay waste to infections and keep us alive ... for a least a while. People who suffer from immune system deficiencies struggle with all sorts of problems.
That is the plight of zombies, too, because they have no immune systems to speak of. With their ample fleshiness, zombies are a perfect breeding ground for untold numbers of bacteria, fungi and viruses that would make short work of their hosts, devouring them from the inside out. And seeing as how they're zombies, that's probably a good thing.