Streptococcal infections are caused by streptococcus bacteria. There are several different strains of streptococcus - the most common are streptococcus A and streptococcus B.

Most infections caused by group A streptococcal bacteria are skin and throat infections, including:

• strep throat (sore throat),
• impetigo (infection of the skin producing pus-filled blisters),
• cellulitis (infection of the skin, and the fat and tissues that lie immediately beneath it),
• erysipelas (inflammation of the upper layers of the skin),
• tonsillitis (severe throat infection), and
• scarlet fever (infection causing sore throat, fever and rash).

Less commonly, group A streptococcal bacteria can get into the blood, deep muscle or fat tissue and cause what are known as invasive streptococcal infections, including:

• bacteremia (infection of the blood),
• endocarditis (infection of the heart lining),
• meningitis (inflammation of the tissues that surround the brain and spinal cord),
• peritonitis (inflammation of the lining of the intestinal tract),
• urinary tract infection (infection of the bladder or urethra),
• necrotising fasciitis (death of tissue under the skin), and
• streptococcal toxic shock syndrome (infection causing low blood pressure and injury to organs such as the kidneys, liver and lungs).

Group B streptococcal bacteria usually cause infections in newborn babies, such as pneumonia (lung infection), septicemia (blood poisoning) and meningitis. Much less often, group B streptococcal bacteria can cause diseases in adults, such as urinary and blood stream infections.

Some people are carriers of streptococcal bacteria on their skin or in their throat, vagina, bladder or rectum. Being a carrier means that they have the bacteria but do not develop an infection.

Some infections caused by streptococcal bacteria such as strep throat are very common. However, the serious invasive infections are much less common.