Cellulitis is an infection of the skin, caused by bacteria. Very often, the face is affected. The disease should not be confused with cellulite. Cellulitis is caused by bacteria, usually staphylococcus or streptococcus, which can get through a break in the skin. These breaks can be caused by a wound, cracks in the skin, insect bites, or problems with the bodies immune system. The infection causes the skin to become red, hot and painful. It can also cause a fever. It is usually treated by taking antibiotic drugs.