High blood sugar from diabetes can affect the body's immune system, impairing the ability of white blood cells to come to the site of an infection, stay in the infected area, and kill microorganisms. Because of the buildup of plaque in blood vessels associated with diabetes, areas of infection may receive a poor blood supply, further lowering the body's ability to fight infections and heal wounds.

People with high blood sugar from diabetes can be more severely affected by common infections, such as influenza and pneumonia caused by Streptococcus pneumoniae. This is why immunizations for influenza (the flu) and pneumococcal disease are recommended for people who have diabetes.

And people who have high blood sugar from diabetes are more likely to be infected with unusual organisms, such as Gram-negative bacteria or fungi.

Foot infections are common in people who have high blood sugar from diabetes. Nerve damage (neuropathy) combined with poor blood supply to the feet puts people who have high blood sugar from diabetes at high risk for infected foot ulcers.

Other infections for which people with high blood sugar from diabetes are at increased risk include:

• Yeast infections on the skin and in the urinary tract.
• Severe infection of the outer ear with the bacteria Pseudomonas aeruginosa. This infection is called malignant external otitis. It causes severe ear pain and drainage from the infected ear.
• Extremely serious infection with the fungus Rhizopus oryzae, which most commonly affects the sinuses and lungs. This infection is called zygomycosis or mucormycosis. It's a complication of diabetic ketoacidosis.