Cellulitis is caused by a bacterial infection. A beta-haemolytic streptococcus (strep) and Staphylococcus aureus (staph) are the most common bacteria involved. More infections these days are due to strains of bacteria that are resistant to antibiotics, such as methicillin-resistant Staphylococcus aureus (MRSA).

The infection usually occurs when bacteria enter the skin through an ulcer, cut, a scratch or an insect bite. It can also happen when there is already a skin problem like eczema, psoriasis, scabies or acne, or after surgery. However it can occur without any visible damage to the skin. A common cause of cellulitis is scratching the skin with fingernails that carry an infection.

Some people are more likely than others to get cellulitis. People are more likely to get cellulitis if they have a skin condition that makes them itchy and are more likely to scratch.

People are also more likely to get cellulitis if they have swelling in an arm or leg such as with lymphoedema. have poor circulation or have a weak immune system. Having diabetes can put a person at higher risk of cellulitis.

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