Answer:
Its fitness would be lower than other strains that lack resistance because it is spending energy that the resistant strains can use for other purposes, such as reproduction.
The second option, but please correct me if I'm wrong.
The body is made to adapt; so it tries to adapt to its current environment. The breathing rate increases to help add more oxygen into the system, slowly boosting endurance as a survival mechanism from ages ago when we actually had to outrun predators. It was likely more efficient back then.
