Streptococcal bacteria are spread by person-to-person contact with someone with an infection or someone who carries the bacteria on their skin. A person who has developed a streptococcal infection is more likely to spread the bacteria than someone who is a carrier, as carriers are much less contagious.

Streptococcal bacteria can also spread in droplets from the nose or throat of someone with an infection.

Occasionally, streptococcal bacteria can enter the body in food contaminated with the bacteria - usually milk and milk products, and eggs.

Many people who come into contact with streptococcal bacteria will not develop an infection. Of those that do, most will develop a minor throat or skin infection. Very few will develop an invasive infection.

Invasive streptococcal infections develop when streptococcal bacteria get past the body's natural defenses. This can happen when a break in the skin allows the bacteria to get into deeper tissue. Health conditions that reduce immunity to infection make invasive infections more likely, so people with chronic illnesses like cancer, diabetes and kidney disease, and those who use medications such as steroids, are at greater risk.

Group B streptococcal infection of newborn babies can be divided into two types:

• Early-onset infection - this is infection that develops in the first week of life. Early-onset group B streptococcus infection is usually associated with pregnancy and is not dealt with here.
• Late-onset infection - this is infection that develops between the 7th and 90th day of life. The source of infection is usually streptococcal bacteria that the baby has come into contact with while in hospital.