These worms hatch out in dead bodies and, if buried, squirm their way to the surface, where they are quickly eaten by birds When eaten, the worms hatch out tiny versions of themselves which cross the blood-brain barrier and take over the bird's mind. They are driven by hunger and the bird in itself will not be enough to feed them through their entire lifespan. So they drive the bird to it's death by wolf or bear or giant spider. In eating them, the spider or other creature itself falls victim to the infection, and is itself taken over by the worms and sent out to attack humans or other animals to provide new hosts for the worms to feed on.
If a human is wounded by one of these possessed creatures, he or she is infected by the worms, but in the short term may not know it, as there are no initial signs except for the wound getting itchy and infected. At this stage healing magics or a mixture of herbs can kill the worms before they can do any more damage, but often the infection is put down to being caused by scratching or by dirt in the wound.
Within an hour or two the victim has a strong headache and an urge to drink a lot. Not long afterwards, the worms infect the brain. Some are aware of themselves being infected and may scream for help, or warn people to get away from them. Others are taken over before they can do anything, and the infected people then lash out at those around them, trying to bite as well as to punch, kick or use weapons. Spells however are beyond the ability of the infected Atissi Zombie, for that is what the victim has become. Immune to pain, they generally have to be hacked to pieces or burned to destroy them.
Whilst zombies created by necromancers do what the necromancer tells them and are not hostile unless ordered to be, having no will of their own, zombies created by Atissi worm infection have a mindless urge to spread the parasite within their worm-filled bodies. They only last for a couple of days, as after that the worms have eaten their muscles to the point that they cannot move, but in that time one zombie can infect a lot of people who in turn infect others, creating a zombie epidemic which if not checked quickly, can become a major menace to the whole region.
When PCs come across a large number of fierce zombies who attack on sight, they are more likely to have been infected by the worm then deliberately created by a necromancer.Creating a zombie needs a lot of proper preperation and powerful magic, and few necromancers have the power to create a large number of them. And the few who do are often high ranking nobles who wish to keep their powers a secret, and so may create a handful of zombies as slaves or assassins, but do not want to go to the great time and trouble to create a zombie army which will reveal them as necromancers to everyone in the region,create panic and generaly disrupt everything.
Once they have eaten the bodies, the worms pupate and turnm into blue beetles which fly off, leaving a husk behind. After mating, they bite something or someone else and the cycle continues.