Erythema is when skin gets red.  This could be due to lots of things: rash, inflammation, injury or infection. Skin looks red because the capillaries in the skin are increasing blood flow in response to a stimulus.  Increase in blood flow can bring more nutrients to the area, clear away debris, and bring extra immune factors to an infection. 

Brandt R Gibson, DPM
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Podiatrist, Neuropathy Doctor, Father of 11 and Founder of Mountain West Foot & Ankle Institute