Why does biodiversity of pathogens cause MSRA and flesh eating disease?

Answer 1

These infuriatingly clever bacteria have adapted to protect themselves from destruction by many types of antibiotics. Fortunately, it caught early and treated aggressively, with both MRSA and necrotizing fasciitis, there are still some antibiotics that can often combat it (e.g., Bactrim, Septra).

Unfortunately, it may just be a matter of time (particularly if the frightening trend of over inapporpriately prescribing antibiotics to "treat" viral infections continues) until these organisms have adapted themselves to the point that there are NO antibiotics that will kill eliminate them.

Bacteria: 1
Humans: 0

Answer 2

MRSA, methlocillyn resistant staph aureus, was a result of too many people not finishing their antibiotics and the bacterium which normally lives on our skin became resistant to all antibiotics.
Flesh eating disease is the result of infection by an anaerobic organism, a whole other ball of wax.
So it isn't really biodiversity that CAUSES these things, it just makes it possible for the organism to exist.