If a human drinks alcohol, the alcohol itself isn't going to the human's brain, so it's not going to kill the Yeerk unless the human totally binges and dies of alcohol poisoning or something...but if you dunk a Yeerk in isopropyl alcohol, or hell, even a beer? I don't know...it would probably absorb pretty quickly, and either kill it, or make it very drunk. Isopropyl would probably kill it more than a beer, I'd imagine x3 I toyed with the idea of naturally occurring alcohol (yeast/fermentation plus fallen fruit or dead things in Yeerk pool water somewhere, stagnant?) on the Yeerk homeworld...because I think waaaaaay too much about this crap x3