Meningitis can be caused by bacteria or a virus. While bacterial meningitis is more dangerous, viral meningitis is more common and runs a milder course.

Bacterial meningitis causes

There are currently a number of bacteria that can lead to meningitis. Some of these include:

• Neisseria meningitidis bacteria or Meningococcal bacteria - There are several different types of meningococcal bacteria called groups A, B, C, W135, Y and Z. At present there is a vaccine available that provides protection against group C meningococcal bacteria. Most cases of meningococcal meningitis, however, are caused by the group B bacteria.
• Streptococcus pneumoniae bacteria or pneumococcal bacteria – These bacteria tend to affect babies and young children and the elderly because their immune system is weaker than other age groups.
• Those who have a CSF shunt or have dural defects are likely to get meningitis caused by Staphylococcus
• Patients having spinal procedures (eg spinal anaesthetia) are at a risk of meningitis caused by Pseudomonas spp.
• Syphilis and Tuberculosis leading to meningitis as well as fungal meningitis are rare causes but are seen in HIV positive individuals and those with a suppressed immunity.

According to age group of the patient the most likely bacterial causes of meningitis include:

• In new-borns - Pneumococcal bacteria or group B streptococci, Listeria monocytogenes, Escherichia coli
• Infants and young children - H. influenzae type b, in children less than 4 years and being unvaccinated raises risk of meningitis due to Neisseria meningitidis, Streptococcus pneumonia
• Older children and adults - S. pneumoniae, H. influenzae type b, N. meningitidis, Gram-negative bacilli, staphylococci, streptococci and L. monocytogenes.
• Elderly and those with a suppressed immunity - S. pneumoniae, L. monocytogenes,tuberculosis (TB), Gram-negative organisms
• After head injury or infection acquired after a hospital stay or procedure - includes infection with Klebsiella pneumoniae, E.coli, Pseudomonas aeruginosa, Staphylococcus aureus

Transmission of infection

The meningococcal bacteria that cause meningitis are usually spread through prolonged, close contact. Spread is possible by being in proximity of an infected person who passes on the bacteria by sneezing, coughing, kissing, sharing personal possessions like toothbrushes, cutlery, utensils etc.

Pneumococcal bacteria are also spread by close contact with an infected person and by coughing, sneezing etc. However, in most cases they only cause mild infection, such as a middle ear infection (otitis media). Those with a poor immune system may develop a more severe infection such as meningitis.

Viral meningitis causes

There are several viruses that may lead to viral meningitis. Vaccinations against many of these viruses have led to the decline in the incidence of several viral meningitis cases. For example measles, mumps and rubella (MMR) vaccine provides children with immunity against mumps, which was once a leading cause of viral meningitis in children.

Viruses that may cause meningitis include:

• herpes simplex virus – this may lead to genital herpes and cold sores
• enteroviruses – stomach flu virus - these have been responsible for causing polio in the past as well
• Mumps virus
• Echovirus
• Coxsackie virus
• Herpes zoster virus
• Measles virus
• Arbovirus
• Influenza virus
• HIV
• West Nile virus

Transmission of the virus

Viral meningitis infection may be spread by close contact with the infected person and being exposed when the person sneezes and coughs.

Hand washing after they are contaminated with the virus – for example, after touching a surface or object that has the virus on it can prevent the spread.

Other causes of meningitis

Other causes of meningitis include:

• Fungal meningitis – Caused by Cryptococcus, Histoplasma and Coccidioides species and seen in AIDS patients
• Parasites causing meningitis – includes examples of eosinophilic meningitis caused by angiostrongyliasis
• Other organisms like atypical tuberculosis, syphilis, Lyme disease, leptospirosis, listeriosis and brucellosis, Kawasaki's disease and Mollaret's meningitis
• There may be no infection and only inflammation of the meninges leading to non-infective meningitis. This is caused by tumors, leukemia, lymphomas, drugs and chemicals given spinally or epidurally during anesthesia or other procedures, diseases like Sarcoidosis, Systemic lupus erythematosus and Behçet's disease etc.

Risk factors of meningitis

Risk factors of getting meningitis include:

• Those living in close quarters like schools, colleges, military base, day care centers, student housings etc. are more at risk of getting meningococcal infections.
• Those with CSF shunts placed in their brain for another pathology
• those with defects in the dura
• use of spinal procedures (eg spinal anaesthetics)
• diabetics
• those with bacterial endocarditis
• alcoholism and liver cirrhosis
• intravenous drug abuse
• renal insufficiency
• thalassemia
• cystic fibrosis
• hypoparathyroidism
• splenectomy
• sickle cell disease