Cellulitis is usually a bacterial infection involving the inner layers of the skin. It specifically affects the dermis and subcutaneous fat. Signs and symptoms include an area of redness which increases in size over a few days. The borders of the area of redness are generally not sharp and the skin may be swollen. While the redness often turns white when pressure is applied, this is not always the case. The area of infection is usually painful. Lymphatic vessels may occasionally be involved, and the person may have a fever and feel tired.

The legs and face are the most common sites involved, although cellulitis can occur on any part of the body. The leg is typically affected following a break in the skin. Other risk factors include obesity, leg swelling, and old age. For facial infections, a break in the skin beforehand is not usually the case. The bacteria most commonly involved are streptococci and Staphylococcus aureus. In contrast to cellulitis, erysipelas is a bacterial infection involving the more superficial layers of the skin, present with an area of redness with well-defined edges, and more often is associated with a fever. The diagnosis is usually based on the presenting signs and symptoms, while a cell culture is rarely possible. Before making a diagnosis, more serious infections such as an underlying bone infection or necrotizing fasciitis should be ruled out.

Treatment is typically with antibiotics taken by mouth, such as cephalexin, amoxicillin or cloxacillin. Those who are allergic to penicillin may be prescribed erythromycin or clindamycin instead. When methicillin-resistant S. aureus (MRSA) is a concern, doxycycline or trimethoprim/sulfamethoxazole may, in addition, be recommended. There is concern related to the presence of pus or previous MRSA infections. Elevating the infected area may be useful, as may pain killers.

Potential complications include abscess formation. Around 95% of people are better after 7 to 10 days of treatment. Those with diabetes, however, often have worse outcomes. Cellulitis occurred in about 21.2 million people in 2015. In the United States about 2 of every 1,000 people per year have a case affecting the lower leg. Cellulitis in 2015 resulted in about 16,900 deaths worldwide. In the United Kingdom, cellulitis was the reason for 1.6% of admissions to a hospital.