What is the flesh eating bug (necrotising fasciitis)?

Several: There are several bugs and combinations that can cause this problem.

Many bacteria!: Necrotizing fasciitis ("flesh-eating disease") is a rare tissue infection usually caused by strep a, but now often mrsa--methicillin-resistant staph aureus bacteria, or multiple other bacteria. This is not a single "super-bacteria" that causes flesh-eating infections (though MRSA is now more common), but any tissue infection with one of several bacterial causes that can spread and destroy rapidly.