Possibilities
Alright, this has really been bothering me over the last week or so. Parasites like Trypanosoma and Toxoplasma gondhii can go cross species. Now I admit that Trypanosoma can't do zombies by itself. The first sign of it is zombie like symptoms. Then it causes the suffering patient (animal/human) to slip into a coma and eventually die. With the immune system in such a weakened state, it would be easy for a viral infection to take place. Toxoplasma Gondhii is another parasite that causes some concern for me. It subtly effects the brain, causing it's (animal/human) host to do some strange things. In rats, the parasite would cause the host to seek out cats so it would be eaten and the parasite could then finish it's lifecycle. In humans it causes pshycological imbalances like schizophrenia and paranoia. A lot of humans are already carrying this. It also causes it's human host to think others with it as beautiful. The human host would revert to natural instinct. Meaning that this type of zombie isn't dead and can be cured with the proper antipsychotics as long as the brain suffered no severe damage.

Mutation of a parasite
Parasites mutate better than most viruses. The link below is an article I read about the mutation of the malaria parasite. http://www.kaisernetwork.org/daily_reports/print_report.cfm?DR_ID=46682&dr_cat=4
The mutation thought can go from bad to worse. Think of toxoplasma causing the irreversible damage to the brain or becoming immune to the antipsychotics.How about an actual parasite that causes the death and reanimation of a lifeform? The host would consume whatever flesh or blood the parasite needed to stay alive and the parasite could in turn cause infection of other hosts by breeding.

-Thoughts of gladimnotdaonly1, if anyone else shares a parasite theory please be my guest. It's good to know all the possibilities.

Fictional zombie parasites

Headcrab zombies

Undoubtably the first fictional Parasitic Zombie was the headcrab zombie from Valve's 1998 game, Half-Life. Roughly the size of a large watermelon, the standard headcrab transforms it's prey into a zombie and feeds off it's face until killed or a better host comes along. The transformation takes seconds and the prey is killed almost instantly. Unfortunatly for the prey, they partialy survive and can be heard yelling backwards, begging to be killed. Another species is the fast headcrab. Their transformation takes longer as they have to strip their prey of their skin, some flesh and all organs. What distinguishes the Fast Zombie from other headcrab zombies is it's ability to climb and move quickly, thus being able to confuse and surprise its prey. Fast Zombies also make loud wails while attacking and can be heard babbling while idle. Fast Zombies stand on two legs while slashing with its claws or standing up, but move on all four legs while running or climbing. Fast Zombies will sometimes "play dead" in order to surprise unsuspecting prey that come to investigate its "corpse". Like regular headcrabs, poison headcrabs have a beak and fangs surrounding the mouth. It also makes a distinctly sinister chirp at range and a hissing/rattling sound similar to that of a rattlesnake's tail when it detects a viable host. When a it gets hold of a host, the mutations that take place produce a bloated, slow-moving poison zombie. Poison Zombies are bloated, reddish purple, slow-moving menaces. They are almost always seen carrying an additional 3 or 4 poison headcrabs on its back.

Flood
The Flood are an extragalactic species from Bungie's 2001 game, Halo, bent on consuming all sentient life throughout the entire universe. In regards to every outbreak of the Flood, there exists three or four stages the parasitic organism undergoes, which are feral, coordinated and interstellar with the possibility of another higher stage present and intergalactic. In the feral stage, the Flood make use of the four basic forms: Infection, Combat, Carrier and a proto-Gravemind form (Brain Form). Once a viable Gravemind has developed, the Coordinated stage begins with the numbers of forms growing rapidly. The Interstellar stage develops once the Flood have acquired sufficient technology and do not possess any further known evolutionary changes. The Intergalactic stage is the last known stage of the Flood, once it has consumed all sentient life in a galaxy (and by extension, gained complete understanding of any technological and scientific knowledge that was present), it will have reached a complete critical mass, which must depart to other galaxies, possibly by either entering a dormant state or by utilizing said gained knowledge and technology, for further replication.

Real Zombie Maker

this only works on grasshoppers and crickets but it shows what can happen when you've got worms

and in case there's anyone that stupid here NO suicide is not normal behavior for crickets it's caused by that worm you see at the end

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Kuthiani: I've held onto this theory for a while: But what about spacebourne parasites? Not to say that I believe in alienkind, but IF I were attacking another planet, I wouldn't go to the planet like Rambo and start annihalating things. I would take over it subtley with some sort of parasitic creature that inhabits the host and takes away their free-will, thus creating chaos. Maybe a stupid theory, but there would be definate danger in the fact that they would probably be rage zombies. Or worse...