Bacterial meningitis
There used to be a large number of different types of bacteria that could cause meningitis, but thanks to vaccination programmes, this number has been reduced.
However, there are a number of bacteria for which no effective vaccines have yet been developed. These are:
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neisseria meningitis bacteria, which are often referred to as meningococcus bacteria (athough a vaccination known as the MenC vaccine does exist for a certain subtype of meningococcus bacteria), and
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streptococcus pneumoniae bacteria, which are often referred to as pneumococcus bacteria.
Pneumococcus bacteria tend to affect babies and young children as their immune system has not built up an immunity to these bacteria.
The meningococcus bacteria can be spread through sneezing, coughing and sharing utensils or personal possessions such as a toothbrush or cigarette.
As most people, especially adults over the age of 25, have a natural immunity to the meningococcus bacteria, most cases of bacterial meningitis are isolated.
However, there is a chance of a small outbreak of cases occurring in environments where a lot of young people live in close proximity, such as a boarding school, university campus, military base or student housing.
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 MMR vaccine provides children with immunity against mumps, which used to be a leading cause of viral meningitis in children.
However, there are still a number of viruses that can cause viral meningitis. These are:
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enteroviruses - these are a group of viruses that usually cause only a mild stomach infection (though in the past they did also cause polio), and
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the herpes simplex virus - which can cause genital herpes and cold sores.
What happens during a meningitis infection?
In the majority of meningitis infections, bacteria or viruses spread through the blood. An infection can begin in one part of the body, such as the throat or lungs, and then move through the tissue into the blood.
Usually the brain is protected from infection by the blood-brain barrier, a thick membrane that filters out impurities from blood before allowing it into the brain.
However, in some people, for reasons that are not entirely clear, the infection can breach the blood-brain barrier and infect the meninges (brain membrane). The immune system will respond to this infection by causing the meninges to swell in an attempt to stop the spread of infection.
Unfortunately, the swollen meninges can then damage the brain and nervous system. Bacteria or viruses can also infect the cerebrospinal fluid (CSF), which is a fluid that surrounds and supports the brain and spinal column. An infection of the CSF can further increase the swelling of the meninges and lead to increased pressure within the skull onto the brain. This is known as intracranial pressure.