There are more than 90 different strains of S. pneumoniae, and some are much more likely to cause serious infection (virulent) than others.

Some strains can be easily killed by the immune system, while others are resistant and likely to cause a more serious infection.

It's thought that between eight and 10 strains are responsible for two-thirds of serious infections in adults, and most cases in children.

How the bacteria is spread

S. pneumoniae enter the human body through the nose and mouth, and an infection can be spread in the same way as a cold or the flu. This can be through:

• direct contact – for example, when someone coughs or sneezes, tiny droplets of fluid that contain the bacteria are launched into the air and can be breathed in by others
• indirect contact – for example, if infected droplets of fluid are transferred from someone's hand to a door handle, someone else who touches the handle may become infected with the bacteria if they then touch their nose or mouth

It's important to emphasise that pneumococcal infections are far less contagious than a cold or flu. This is because most people's immune systems are able to kill the bacteria before they have the opportunity to cause an infection.

Outbreaks of pneumococcal infections can sometimes occur in environments where there are many people who have poorly functioning immune systems, such as in children's nurseries, care homes for the elderly and homeless shelters.

Risk factors

People with a weakened immune system, either due to their age or general health, are particularly at risk of developing a pneumococcal infection.

The bacteria can move from their throat to other parts of their body, such as the lungs, the blood or the brain. If this occurs, a more serious infection can develop.

The pneumococcal vaccine may be considered for people at higher risk, including:

• children under two years of age
• adults over 65 years of age
• anyone between the ages of two and 65 with a long-term health condition

You're considered to be at a higher risk of pneumococcal infection if you:

• have a weakened immune system as a result of a health condition such as HIV or AIDS
• are receiving medical treatment that is known to weaken the immune system – such as chemotherapy or corticosteroids
• have a history of spleen disease or dysfunction
• have a chronic respiratory disease – such as chronic obstructive pulmonary disease (COPD)
• have coronary heart disease or have previously had a heart attack
• have chronic kidney disease or alcohol-related liver disease
• have diabetes and need to take insulin
• wear a type of hearing aid called a cochlear implant – people who use these have a slightly increased risk of developing meningitis, but the reasons for this are unclear
• have spinal damage that has caused their cerebrospinal fluid (a fluid that surrounds the brain and spine) to leak