Viraj's Weblog

Ever wonder how the rabies virus achieves its evolutionary goals? What's the point of making the host crazy and rabid? Here's an excerpt from Carl Sagan's Shadows of Forgotten Ancestors:

Of course, predators need not be bigger than their prey. Disease microbes can be formidable predators - not only attacking and eventually killing the organisms that bear them, but also taking over their hosts, changing their behavior to spread the disease microorganisms to other hosts. One of the most striking examples is the rabies virus. On being injected into the bloodstream of a placid, people-loving dog, they head straight for the limbic system of the dog's brain, where the control buttons for rage reside. There, they set about converting the poor animal into a marauding, snarling, vicious predator that now bites the hand that feeds it. Rabid animals are afraid of no one. At the same time, other rabies viruses are dispatched to inactivate the nerves for swallowing, to put the saliva-manufacturing machinery into overdrive, and to invade the saliva in huge numbers. The dog is furious, although it has no idea why. A pawn of the viruses within it, it's helpless to resist the impulse to attack. If the attack is successful, the viruses in the dog's saliva enter the bloodstream of the victim through the lesion or laceration, and then set about taking over this new host. This process continues.

The rabies virus is a brilliant scenarist. It knows its victims, and how to pull their strings. It circumvents their defenses - infiltrating, outflanking, accomplishing a coup d'etat within beings so much larger, you might have thought them invulnerable.

It's rather amazing that the rabies virus evolved in such a way to see this big picture.