Richard Dawkins dedicated his entire book The Extended Phenotype (which is an excellent read if you get the chance) to elaborate on the idea that your genes may express themselves not only as adaptations in your organism (a certain gene for longer arms or whatever), but possibly as adaptations in other, physically distinct organisms, via exactly the same mechanisms.
In his book, Dawkins alludes to the concept of parasites' intermediate and definite hosts, where a definite host is the host that eventually nourishes the parasite, and the intermediate host is only the means through which the parasite infects its definite host.
Although the book is riddled with examples, here's one that I think is especially instructive, as it deals with two parasites that exert different zombifying effects on the same host:
[Bethel and Holmes] studied two species of acanthocephalan worms, Polymorphus paradoxus and P. marilis. Both use a freshwater 'shrimp' (really an amphipod), Gammarus lacustris, as an intermediate host, and both use ducks as the definitive host. P. paradoxus, however, specializes in the mallard, which is a surface-dabbling duck, while P. marilis specializes in diving ducks. Ideally then, P. paradoxus might benefit by making its shrimps swim to the surface, where they are likely to be eaten by mallards, while P. marilis might benefit by making its shrimps avoid the surface.
Uninfected Gammarus lacustris tend to avoid ligth, and stay close to the lake bottom. Bethel and Holmes noticed striking differences in the behaviour of shrimps infected with cystacanths of P. paradoxus. They stayed close to the surface, and clung tenaciously to surface plants . . .
Shrimps infected by cystacanths of the other species, P. marilis, however, do not hug the surface. In laboratory tests they admittedly sought the lighted half of an aquarium in preference to the dark half, but they did not orient positively towards the source of light: they distributed themselves randomly in the lighted half, rather than at the surface. * The Extended Phenotype, Dawkins, p. 216-217
I realize you may not accept this 'shrimp' as a high order animal, but I think it is a useful example to sell in the idea of applying this zombiefying effect on one host, so as to be able to reach another. I wanted to sell in this idea, to see that this effect is not so uncommon, once you start to view the events in that light:
Dogs, when affected with rabies, will be driven to bite other dogs (well, other animals, really). A bland interpretation would be that the virus irks the dogs, and they're getting really aggressive about that. But consider that this tendency to bite others is the primary means through which the virus spreads. Could we not then agree that there is a direct appreciable selective pressure for this virus to exert this effect?
We know that this tendency is observed in rabies infected animals, and we can see that there is great evolutionary advantage to the rabies virus in it. I don't know what kind of scientific evidence you would need to be able to say that spreading the virus is the reason why the animals behave in this way, but I would certainly argue that that's the most plausible explanation.
Moving on along that line; we know that humans produce snot as an outlet for foreign bodies to leave our own body. But given that we have this mechanism up and running, wouldn't there be a huge selective advantage for the common cold virus to cause us to sneeze while in this condition? At this end, it becomes a bit sketchy, but I'd say drawing the line is single-handedly a matter of interpretation.
