There are more than 90 different strains of Streptococcus pneumoniae (S. pneumoniae) and some are much more likely to cause serious infection (virulent) than others.
Each strain of bacteria is classified by the type of complex sugar molecules that make up the outer shell of the bacteria.
Exactly how the outer shell is constructed is one factor that determines how virulent each strain is. Some strains have a weak and flimsy outer shell, which means the bacteria can be easily killed by infection-fighting white blood cells.
Other strains have a very tough outer shell, which makes the bacteria resistant to white blood cells and likely to cause a more serious infection.
It is thought that 8-10 strains are responsible for two-thirds of cases of serious infections in adults and most cases in children.
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 sneezes or coughs, 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 mouth or nose
It is important to emphasise that pneumococcal infections are far less contagious than a cold or the 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 a poorly functioning immune system, such as in children's nurseries, care homes for the elderly and shelters for people who are homeless.
In most cases, S. pneumoniae bacteria do not cause a problem because the immune system prevents them moving from the throat to other parts of the body.
However, in people with a weakened immune system, either due to their age or general health, 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.
People who are particularly at risk from developing a pneumococcal infection are listed below.
- children under two years of age
- adults over 65 years of age
- those with a weakened immune system as a result of a health condition such as HIV or AIDS
- those who are receiving medical treatment that is known to weaken the immune system, such as chemotherapy or corticosteroids
- those with a history of spleen disease or dysfunction (the spleen is an organ that filters the blood and plays an important role in fighting infection)
- those with a chronic respiratory disease, such as chronic obstructive pulmonary disease (COPD)
- those with coronary heart disease or who have previously had a heart attack
- those with chronic kidney disease or chronic liver disease
- those with diabetes who need to take insulin
- those who 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)
- those who are addicted to alcohol or regularly misuse alcohol (alcohol misuse is known to weaken the immune system)
- those who are currently infected by the influenza virus (this applies to both seasonal flu and swine flu)
- those who smoke cigarettes or other tobacco products
- those who are living in poverty (many of the factors related to poverty, such as poor diet and living in unhygienic and overcrowded environments, increase a person's risk of developing a pneumococcal infection)
- Those with spinal damage that has caused their cerebrospinal fluid (a fluid that surrounds the brain and spine) to leak