Tolerance builds up during your early drinking. You notice it takes more to get you drunk as you progress. In late stages of alcoholism, it reverses, and can do so drastically. One drink an you can be a drooling idiot. I actually got to see this in action as a bartender in my early 20s. One guy would come into the bar and order a highball. He was personable, alert, and normal. I would pour his drink and then I would move down the bar taking care of customers, by the time I came back to his end of the bar, he would be blotto, spitting, and incoherent. This may have occurred after his second drink; It was a long time ago when I was a bartender, so I don't remember exactly. I'd never seen someone lose it that fast, and I mentioned it to one of the regulars who explained he was an alcoholic. I was 22, and didn't know much about it. That was my first introduction to wet brained (final stage) alcoholism. I was shocked, not in a horrified way, but shocked in a not understanding way.
Alcoholism is progressive. It gets worse even when you have had a long stretch of sobriety. I've only been told this. I experienced it when I was drinking. It just kept getting worse. When I quit, I never drank again, so the part about getting worse during sober vacations, I've only read about. I never tested those waters.