There are several ways to reanimate the dead, you can use haitian voodoo which is mainly the poison of a puffer fish.
Theres also radiation if delt in small doses (anything to big will kill you). Then theres solarium, not alot is known about solarium but it reanimates the dead and can only be broken down by water.
That about it, apart from possibly a metorite or something like that.
#1 reason that zombification isnt possible, is the brain, once you die your brain loses oxygen which it needs to function, it only takes 8-9 minutes for the brain damage to occur from lack of oxygen, futhermore if you reanimate them they'll just be husk of the former human being they used to be.
The Zombie virus mutates the cells so it does not need oxygen. It is a self feeding organism. Also, that's why it doesn't actually take a day for zombies to reanimate after a death. Usually after the victim dies, he/she reanimates a few minutes, or possibly seconds, after their death.