A skin infection usually caused by bacteria. Bacteria can enter into the skin through a cut or insect bite and spread to deeper tissues causing an infection. If it is not treated with antibiotics, the infection can spread to the blood or lymph nodes. The most common bacteria are staphylococcus and streptococcus species. A particular staphylococcus species called Methicillin-resistant Staphylococcus aureus (MRSA) is becoming a more common cause. People with depressed immune systems as with diabetes, cancer, HIV, chronic steroid use, and autoimmune diseases are more likely to develop cellulitis.