Stem cell research is going to enable people to regenerate decayed organs soon in the future, but the brain is different. The brain decays from the outside in, and the outside of the brain is what contains human empathy and compassion. The brain decays so quickly that unless revived immediately, even with stem cell there's no way that outer layer could be brought back. So this could leave a person with a functioning inner layer, which is pure survival, and no feelings for others. YIKES. 

As anyone who has watched the Simpsons knows, if you ingest the neurotoxins contained in a pufferfish, you'll die. However if rubbed on the skin instead of ingested, the toxins can bring on a sort of zombie state, not actual death, in humans. If someone were able to somehow mass produce and distribute this...shit could get REAL. 

Remember the time some guy attacked and nearly at the face off of a homeless man? Yeah...not much more explanation needed here.

The potential power of disgusting little parasites is pretty outrageous and terrifying. These parasites infect their host, lay eggs and start taking over the brain. So far they've almost exclusively infected insects, but there have been a couple of human instances too...let's hope that quantity remains a small one. 

Sort of like the horsehair parasite, this is another one that so far really only deals with insects, but has terrifying possibilities if it ever mutated to infect humans. This wasp stings its prey in such a deliberate way it ultimately paralyzes them but then gives them the strength to do simple necessary tasks such as walk. It pretty much turns its prey into zombies.