Meningitis can be caused by bacteria or a virus.
Bacterial meningitis
Vaccination programmes have helped to reduce the number of different types of bacteria that can cause meningitis. However, there are currently a number of bacteria for which no effective vaccines have been developed. Some bacterial causes are described below.
Neisseria meningitidis bacteria
Neisseria meningitidis bacteria are often referred to as meningococcal bacteria. There are several different types of meningococcal bacteria called groups A, B, C, W135, Y and Z.
There is a vaccination that provides protection against group C meningococcal bacteria. See the Health A-Z topic about the Men C vaccination for more information.
In the UK, most cases of meningococcal meningitis are caused by the group B bacteria.
Streptococcus pneumoniae bacteria
Streptococcus pneumoniae bacteria are often referred to as pneumococcal bacteria. Pneumococcal bacteria tend to affect babies and young children because their immune system (the body’s defence system) has not built up immunity (protection) to these bacteria.
Spreading the bacteria
The meningococcal bacteria that cause meningitis do not live long outside the body, so they are usually only spread through prolonged, close contact. Possible ways to spread the bacteria include:
- sneezing
- coughing
- kissing
- sharing utensils, such as cutlery
- sharing personal possessions, such as a toothbrush or cigarette
As most people, particularly adults above 25, have a natural immunity to the meningococcal bacteria, most cases of bacterial meningitis are isolated (single cases). However, there is a chance of a small outbreak of cases occurring in environments where a lot of young people live close together. For example:
- a boarding school
- a university campus
- a military base
- student housing
Pneumococcal bacteria are much easier to catch than meningococcal bacteria, and they are spread through coughing and sneezing. However, in most cases they only cause mild infection, such as a middle ear infection (otitis media).
See the Health A-Z topic about Pneumococcal infections for more information about the type of infections that these bacteria can cause.
Viral meningitis
As in the case of bacterial meningitis, vaccination programmes have successfully eliminated the threat from many viruses that used to cause viral meningitis.
For example, the measles, mumps and rubella (MMR) vaccine provides children with immunity against mumps, which was once a leading cause of viral meningitis in children. See the Health A-Z topic about the MMR vaccination for more information.
There are still a number of viruses that can cause viral meningitis. These include:
- enteroviruses – these are a group of viruses that usually only cause a mild stomach infection, although in the past they also caused polio (a condition that can cause paralysis but is now rare as a result of successful vaccination programmes)
- the herpes simplex virus – this can cause genital herpes and cold sores
These viruses can be spread through:
- coughing
- sneezing
- not washing your hands after they are contaminated with the virus – for example, after touching a surface or object that has the virus on it
During a meningitis infection
In most meningitis infections, bacteria or viruses spread through the blood. An infection can begin in one part of the body, such as your throat or lungs, before moving through the tissue and into the blood.
The brain is usually protected from infection by the blood-brain barrier, which is a thick membrane that filters out impurities from the blood before allowing it into the brain.
However, in some people, for reasons that are not entirely clear, the infection is able to pass through the blood-brain barrier and infect the meninges (brain membrane). The immune system responds to the infection by causing the meninges to swell in an attempt to stop the spread of infection. The swollen meninges may then damage the brain and the rest of the nervous system (nerves and spinal cord).
Bacteria or viruses can also infect the cerebrospinal fluid (CSF), which is the fluid that surrounds and supports the brain and spinal cord. An infection of the CSF can cause further swelling of the meninges, leading to increased pressure in the skull and pressing on the brain. This is known as intracranial pressure.
