system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a virus, like rabies, cause a very specific psychological symptom in the brain, like fear of water? Also, what is the evolutionary advantage of virus-induced fears?
gpt
This is a great question!  Rabies is one of the few viruses known that actually causes a behavioral change in the infected animal.  It's really quite amazing, but it all boils down to the virus optimizing it's own transmission.

Rabies gets into an animal (or human) from the bite of another infected animal.  Once injected into the muscle, the virus enters nerve cells and begins moving towards the brain.  This keeps it hidden from the immune system.

Not only does the virus go to the brain, but it also goes to the salivary glands, where it replicates to very high concentrations.  Further, the virus causes the production of large amounts of saliva.

In the brain, the virus causes an inability to swallow ("hydrophobia").  It also causes fear and aggression in the animal.

So, how does all this help the virus?  The virus replicates in the salivary glands.  It causes large amounts of saliva in order to carry the virus.  It prevents the animal from swallowing the virus-filled saliva as well.  Further, the virus causes the animal to be afraid and aggressive, thus more prone to bite.  All of this promotes the efficient transmission of the virus, and thus carries an evolutionary advantage for the virus.  Quite cool!