NF usually affects legs and arms, but can occur anywhere on the body. The pain is usually almost unbearable - totally out of proportion to external signs because the infection begins deep under the skin, spreading along a thin membrane ('fascial plane') near to the muscle. Eventually the thin membrane (fascia) dies, causing necrotising ('death of') fasciitis ('inflamed fascia') and severe deep pain due to swelling and inflammation. As infection rises to the skin surface it causes discolouration which may be pinky-red, bruised or blistery. Very late in the infection numbness and areas of purple/blackness, bloody blisters of dead skin overlie the deeper dead tissue.
NF can be difficult to diagnose because initially there is not much to see and it can cause confusing symptoms. Toxins produced by Streptococci and Staphylococci may cause vomiting and diarrhoea- hence easily misdiagnosed as a 'tummy upset' or food poisoning. Deep pain without any outward change in the skin is easily blamed on a muscle strain.