These are all hijacking machines.
So instead of digging its own hole in the sand and lay down its eggs, the female sacculine barnacle finds it much more fun to adhere to a green crab's back, slowly move on the shell's surface until it finds a joint, and then send its cells invading the body.
It develops until it reaches the underside, where it drills an opening for the male sacculine barnacle to enter. The couple of parasites then make thousands of eggs, castrating the crab in the process and rewiring it to their purpose. What if the crab is a male? Well it seems in this case the parasite secretes a substance which demulitplies the crab's estrogen level. Poor crab shortly starts digging a perfectly fine hole -except of course it is not its eggs it is laying. Ah, ..nature's cruelly unsensitive genius..
I you think that wasn't so cute, rabies is worse. As is well known, this lethal virus most commonly propagates through bites (98% of human infection cases come from dog bites). Once in another mammal's body, the virus completely disdaigns any other organ than the brain, towards which it immediately starts migrating. This can take months, but once it's there all bets are off. In a very real sense this organism knows more about the brain than any living neuroscientist. It also "knows" it can only be passed on by making the host bite as many animals as possible (or rather, those ancester rabies viruses whose dna did not program them to act accordingly did not reproduce) . So it makes the host extremely agressive, with a strong need to bite all living things arounds.
But mind you, that's not sufficient: because saliva is essential in the spreading, the virus has to make the host produce a lot of saliva. His way to do that is to migrate to the salivary glands and crazily stimulate them. The host starts producing plenty of saliva. But wait, what if the host drinks and looses this saliva? Well rabies has "thought" it all, and simply makes it impossible for the host to swallow any kind of liquid..the host finally dies paralyzed, or in the rare documented cases of remission, with severe brain damages.
Finally we come to toxoplasmosis..here's an equally impressive, although slightly more subtle virus. Lethal only if you're a rodent (or so it would seem). I highly recommend this Edge video to get an up-to-date, cutting edge understanding of what is going on.