虽然感染的直接原因是致病微生物，但非开放性外伤可以是感染的始发因素。你大概
不知道NSTI. Necrotizing soft-tissue infection is an infection of the subcutaneous
tissue and fascia associated with sepsis and a possible fatal outcome. Although
the cause of it is not yet understood fully, patients often have a prior
history of a small, trivial trauma, or wound.
第二个问题已有网友作了回答，恕不重复。
我相信你学过医。