Tongue and larynx of rabid dog
People once thought that rabies was caused by a worm in the sublingual salivary glands, because of how tight and swollen they become when rabies is infecting the system. We know now that that’s not true. Rabies is caused by a virus that infects the nervous system. Once it enters the body, it seeks out the peripheral nervous system, and moves along peripheral nerve cells until it reaches the CNS. The virus continues up the central nervous system until it reaches the brain, where it multiplies, causes the extreme symptoms, and kills the victim.
Up at the top of this illustration, you can see the inflamed section of the upper throat. This goes along with the involuntary throat spasms that rabies entails, which prevent the ingestion of any liquids. The spasms are often incredibly painful, and the avoidance of liquids (though not a true fear of them) is why rabies used to be called hydrophobia.
Rabies and Hydrophobia: Their History, Nature, Causes, Symptoms, and Prevention. George Fleming, 1872.