Here's an article about how hyenas and humans have their behavior modified by this common parasite (toxoplasmosis)
It makes hyenas, humans, and mice more "brave" getting the latter eaten more often by cats, which is their carrier.
If you want to find a major potential weakness in theories of consciousness, parasitism is a major hurdle to get over. Parasites regularly turn host metabolism to their own advantage and to the disadvantage of the host (snail flukes make the snail grow thicker shells, for instance). But a big one is that parasites control the "mind" and the behavior of their hosts, often to cause things like getting themselves eaten so as to pass the parasite on to the next phase of the parasite's lifecycle.
Another behavior modification is to force the host to take care of the offspring of the parasite. The cuckoo bird is a particularly compelling example. We might think of parasites as microbes, or fungi (as in the ones that take over ant's brains), or even flukes and worms (intestinal, blood, etc.). The cuckoo lays it's egg in some other bird's nest and then buggers straight off for good. The cuckoo chick pushes all of the host's eggs out of the next, then "hypnotizes" the host into feeding it, even to the point that the host bird is standing on the back of the parasite chick, stuffing food down its throat.
People who work with addictions would not object to the comparison of the substance or behavior acting like a parasite and the addict like a host.
And of course a major tenet of memetics is that memes and memeplexes are parasites of the mind of the individual and of groups of individuals.
So the upshot is that the effect of parasites on the mind can be very subtle, very sophisticated, very profound, and highly variegated. This fact may sweep away many of the aspects of "consciousness" we hold near and dear, motivation, sense of self, perspective, among perhaps many others. Of course, mental illnesses (including addiction) cast these concepts into question as well.