Most cases of cellulitis are caused by an infection of the tissues beneath the skin with either the group A streptococcus or staphlococcus aureus bacteria.
See streptococcal infections and staphyloccocal infections for more information about these types of bacteria.
Cellulitis usually occurs when the surface of your skin is damaged. This damage creates an entry point for the bacteria, allowing them to attack the skin and tissue underneath. A break in the skin may be caused by a:
The break in the skin may be so small that it cannot be easily identified.
Some cases of cellulitis can develop if a wound or other break in the skin is exposed to water that is contaminated with bacteria.
A fungal infection is a much rarer cause of cellulitis. Fungal cellulitis usually only affects people with a severely weakened immune system, such as a person in the final stages of an HIV infection that is not responding to treatment.
Who is at risk?
A number of health conditions can increase your risk of developing cellulitis. These include:
- being obese (excessively overweight)
- having a weakened immune system
- having poorly controlled diabetes
- having circulation problems
- having chickenpox or shingles
- having lymphoedema
- having long-term untreated athlete's foot or fungal toenail infection
- intravenous drug use
- having previous episodes of cellulitis
These are briefly discussed below.
Being obese can cause swelling in your legs, which may increase your risk of developing cellulitis.
Obesity is defined as being very overweight with a body mass index (BMI) of 30 or more. To check your BMI, you can use the healthy weight calculator.
Weakened immune system
Your immune system may be weakened if you have a condition such as HIV and AIDS. Having a weakened immune system makes it harder for your body to fight off infection.
A number of treatments are also known to weaken the immune system. For example:
chemotherapy - a cancer treatment that uses medication to kill cancerous cells
- immunosuppressants (medications that are widely used to treat people who have had organ transplants to prevent their body rejecting the donated organ)
- long-term use of corticosteroids tablets and corticosteroid creams
Poorly controlled diabetes
If you have diabetes that is not adequately treated or controlled, it can weaken your immune system.
Poorly controlled diabetes can also affect your circulation, which can sometimes cause skin ulcers to develop. Skin ulcers are a common entry point for bacteria.
Learn more in diabetes.
Poor circulation can increase your risk of developing skin infections in the places where your body does not have an adequate blood supply.
For example, many people with diabetes have a reduced blood supply to their feet, which makes them more vulnerable to developing cellulitis.
Chickenpox and shingles
Chickenpox and shingles often cause blisters to develop on your skin. Chickenpox (which usually only affects children) and shingles (which usually affects people aged 50 and older) are viral infections caused by the herpes varicella-zoster virus.
If the blisters that occur in chickenpox or shingles are broken or scratched, it can damage your skin and provide an entry point for bacteria.
Lymphoedema is a condition that causes fluid to build up under your skin. It may occur following surgery for some cancers. If your skin becomes very swollen it may crack, creating an entry point for bacteria.
Intravenous drug use
People who inject illegal drugs have an increased risk of developing cellulitis, because poor needle hygiene, such as not sterilising the needle before and after injections, can increase the risk of infection.
Previous episodes of cellulitis
If you have had a previous episode of cellulitis, your risk of having episodes in the future increases.
An estimated 20-30% of people with a previous history of cellulitis will be admitted to hospital again with another cellulitis infection. The average time between a previous and recurring cellulitis infection is three years.
- Chronic usually means a condition that continues for a long time or keeps coming back.
- Immune system
- The immune system is the body's defence system, which helps protect it from disease, bacteria and viruses.
- The liver is the largest organ in the body. Its main jobs are to secrete bile (to help digestion), detoxify the blood and change food into energy.
- Obesity is when a person has an abnormally high amount of body fat.
- Veins are blood vessels that carry blood from the rest of the body back to the heart.