My sister recently had this in her arm. It swelled a lot and luckily my father had the gut feeling to take her to the emergency room. I had never heard of it no

My sister recently had this in her arm. It swelled a lot and luckily my father had the gut feeling to take her to the emergency room. I had never heard of it nor would have thought it would be life threatening! She had mentioned that her workout wasn't very intense, be careful out there.