The meaning of the word, 'fittest', in the theory is not biggest, toughest, strongest, as we usually think of it; it is rather 'that fits best' in the circumstances. For example, there was a case of a type of moth which was basically white common in northern England, though there were also dark ones. When the industrial revolution got under way, smoke from factories tended to blacken surfaces for miles around and the inevitable happened...the white ones stood out against the background, whilst the dark ones did not. Predatory birds simply picked the white ones off easily, but failed even to see the dark ones. They survived because they were more 'fitting' in their environment, not because they were in any way stronger etc.
Our medical advances are now, in effect, part of "nature"...rather like the smoke above.