That an infection could reanimate the dead is preposterous, and it appears more preposterous for every zombie that gets back up and wanders around The Z. How could microbes cause dead cells to not only reactivate, but coordinate movement, digestion, the senses and complex motor abilities?
Because they’re magic microbes. All science has been a cover-up of the obvious and insulting answer that just as life began at the microscopic level, so did the arcane. If you study fossils or present live bacterial cultures, you’ll find they’ve evolved little since the dawn of life. That’s because they mastered it. All they needed was to develop enough magical potential to survive a little better than anyone else. The typical virus either dies out fighting the immune system or dies out when it kills the host. The magic microbe that could magically reanimate its host was much better, and spread its offspring much better, given that zombies shamble around sharing the miracle.
Coincidentally, vampirism is believed to be the work of a more advanced but less photo-resistant microbe. Lycanthropy, though, is just a hoax by guys in hairy suits. All real magic in triclopes, humans and imps can be credited to the plagues that first bred it into us.