Q:

Considered most unbearable = considered very unbearable?

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.

A:

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.

