Alcohol denatures proteins, makes them unfold. Proteins are made up of fatty acids and alcohol is a solvent. My theory is that viruses mutate into shorter protein strands to be less affected by the unfolding, and communicates this to the rest. This makes them an antibody to the original virus, and the new entity affects  the host body less.

I always heard that if you took a drink 3 days before you felt it coming on, you'd never get sick.
And you'd be drunk all the time.