Meningitis can be caused by bacteria or a virus.

Bacterial meningitis

Vaccination programmes have helped reduce the number of different types of bacteria that can cause meningitis.

However, there are currently a number of bacteria for which there currently aren't effective vaccines. 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, W, X, Y and Z.

There's a vaccination that protects against group C meningococcal bacteria. Read more about the Men C vaccination. There's also a quadruple vaccine that provides protection against group A, C, W and Y meningococcal bacteria.

In the UK, most cases of meningococcal meningitis are caused by the group B bacteria. A vaccine for group B bacteria has recently been developed and approved for use, but it isn't currently available on the NHS.

Read more about the new meningitis B vaccine. 

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 systems (the body's natural defence) haven't built up immunity (protection) to these bacteria.

Spreading the bacteria

The meningococcal bacteria that cause meningitis can't survive for long outside the body, so they're usually only spread through prolonged, close contact. Possible ways the bacteria are spread include:

• sneezing
• coughing 
• kissing
• sharing utensils, such as cutlery
• sharing personal possessions, such as a toothbrush or cigarette

As most people, particularly adults aged over 25, have a natural immunity to the meningococcal bacteria, most cases of bacterial meningitis are isolated, single incidents.

However, there's a chance of a small outbreak of cases occurring in environments where many young people live in close proximity. For example:

• boarding schools
• university campuses
• military bases
• student housing

Pneumococcal bacteria are more easily spread than meningococcal bacteria, and are passed on through coughing and sneezing. However, in most cases, they only cause mild infections, such as a middle ear infection (otitis media).

Read more about pneumococcal infections.

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.

However, there are still a number of viruses that can cause viral meningitis. These include:

• enteroviruses – a group of viruses that usually only cause a mild stomach infection, and can be spread through coughing, sneezing or not washing your hands after touching a contaminated surface
• herpes simplex virus – which can cause genital herpes and cold sores 

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, 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) that surrounds and supports the brain and spinal cord. If the CSF becomes infected, it can cause the meninges to become more swollen, leading to increased pressure in the skull and pressing on the brain. This is known as intracranial pressure.