In this sentence, does most mean very, or is it a shorter version of the most? Are both meanings possible here?
Well, the diseases that human beings focus on are the ones considered most unbearable.
In this case "most" can stand in for "the most" and "very" but it could also be thought of to mean "extremely", however to me your excerpt reads:
well, the diseases that human beings focus on are the ones that are considered the worst to bear.