It's not an underlying infection. It's a reinfection, made far more likely by a previous infection, probably because of the lymphoedema around my ankles I retained from the first infection. The blood supply which maintains the immune system is reduced in the swollen areas. The infection is caused by normal skin bacteria which get into the lower layers of the skin through little scratches or insect bites (of which I've endured many this summer). And cellulitis that isn't promptly treated can be life-threatening as it can, and sometimes does, lead to sepsis. Not to be played down.