My buddy Mark says he must have bumped into something hard enough to break the skin, but he does not remember such an event. Anyway, the doctor said he has a skin infection called "cellulitis" ( #TIL: https://www.mayoclinic.org/diseases-conditions/cellulitis/symptoms-causes/syc-20370762 ), so he's on antibiotics. I had never heard of #cellulitis before.