Parasites are even more squiffy, compared to bacteria and viruses, to cause a zombie outbreak. Why? Because parasites aren't TRYING to take over you; like any stowaway, a parasite's goal in life is to survive as long as possible without detection or intervention from its host, and mind control isn't exactly the best way to accomplish that (animals instinctively know when something's up, and as a human being, I think that you'd stay clear of a neighbor who has a dazed look on his face and blood on his teeth).
When a parasite exerts control over an organism, it causes the host to be isolated from the group and prevents it from continuing its life cycle by infecting other hosts; in fact, the ant parasite causes the ant to physically isolate itself to be eaten and thus killed. Most of those parasites use ants and other animals to get into their ACTUAL host, because they have no other way to infect them; once inside those hosts, they do not cause death or mind control, and passively infect other hosts. The opportunistic toxoplasma, again, infects with a minimal impact on its host (which is interchangable with a cat or dog, and thus doesn't represent anything out of the ordinary). However, the implications of an infection aren't really confirmed; it's a correlation, but correlation doesn't equal causation.
A parasitic infection of humans implies that there's some organism that it can't reach otherwise. Within natural habitats that we HAVEN'T messed up, there's literally hundreds of species and dozens of animal types out there to infect, as opposed to the rather biologically impaired human (without tools or higher thinking, we are pretty much walking meat, and meat exists in more accessible areas outside of an urban city than within it)
The scary part about parasites is having something inside of you that could change who you are as a person, but then, that has less to do with zombies and more to do with fear of insanity.