"As Well"

Why does this mean "also" in the English language? What does "also" or "too" have to do with "as well." As well should describe health or something. Just a thought to ponder. I always try to literally translate phrases in other languages... bad habit but sometimes interesting. Often I am confused, "why do say this when they mean that?!" Well, here is an example in English that I've been unconscious of for 25 years!