Meningitis is an inflammation of the protective membranes which cover the brain and spinal cord, known collectively as the meninges, which can be caused by bacterial, viral or fungal infections. Without treatment, bacterial meningitis can be fatal, accordingly the condition is considered a medical emergency and prompt medical attention should be sought. 

Bacterial meningitis occurs when bacteria enters the bloodstream and migrates to the brain and spinal cord. It may also occur when bacteria directly invade the meninges, as a result of an ear or sinus infection or trauma to the skull. 

Bacterial and viral meningitis are contagious, but neither are as contagious as the common cold or flu. Both can be transmitted through droplets of respiratory secretions during close contact such as kissing, sneezing or coughing on someone, but cannot be spread by only breathing the air where a person with meningitis has been.

Bacterial meningitis can be life-threatening and requires urgent medical attention. If diagnosed and treated promptly, many can make a full recovery, however some who contract bacterial meningitis may be left with life-long disability or for others, it may result in death. It is important to note, that there is no vaccine to protect against all strains of meningitis, so knowing the signs and symptoms and seeking medical attention right away are critical to a good prognosis. 

The only definitive diagnosis for meningitis is often made by analyzing a sample of the patient's cerebrospinal fluid (CSF), which is collected through a lumbar puncture (spinal tap). In people with meningitis, the CSF fluid often shows a low sugar (glucose) level along with an increased white blood cell count and increased protein. Further, gram staining may indicate the bacteria involved in bacterial meningitis.  If meningitis is suspected, aggressive wide-spectrum antibiotic treatment must be administered promptly in order to prevent neurological complications.

The after-effects of meningitis can be severe and include: hearing loss, blindness, memory difficulty, loss of speech, epilepsy, learning disabilities, behavior problems, limb loss and scarring (meningococcal septicaemia), brain damage, paralysis, multiple organ failure and death.

Even with effective vaccines, advanced tools to identify pathogenic bacterium, and potent antimicrobial medication, neonatal meningitis is a significant source of neurological disability around the world. Newborn infants are more susceptible to sepsis and meningitis than any other age groups because of deficiencies in their immune response.

With medical advancement, the rate of mortality from bacterial meningitis among neonates has declined, however, a corresponding decrease in the morbidity rate has not occurred. Morbidity associated to neonatal bacterial meningitis continues to be a significant source of disability. Approximately 50% of neonates will sustain neurological sequelae and developmental disorders.

Neonatal meningitis is often caused by vertical transmission during labor and delivery. It typically occurs in the days following birth and is more common in premature infants than in term infants. Neonatal meningitis may also occur as a result of a nosocomial hospital acquired infection or a community acquired infection.

The risk factors involved in developing neonatal bacterial meningitis include low birth weight, preterm birth, premature rupture of membranes, traumatic delivery, fetal hypoxia, and maternal infection.

In Canada, the leading types of neonatal meningitis include Group B streptococci (GBS), Escherichia coli (E coli) Listeria monocytogenes (Listeria). Moreover additional gram negative enteric bacteria may be responsible including: Klebsiella, Enterobacter, Citrobacter, Salmonella, Proteus, Pseudomonas, and Serratia.

There are many different types of meningitis, however the most common include: 

Pneumococcal meningitis is caused by the pneumococcus bacterium, Streptococcus pneumoniae. The pneumococcus bacterium can cause other serious infections such as pneumonia, blood poisoning and septic arthritis, as well as infections such as otitis media and sinusitis. The pneumococcus bacterium lives in the back of the nose and throat and attacks the human body for unknown reasons. People with weaker immune systems are at a greater risk of contracting this type of bacterial meningitis. Pneumococcal meningitis mainly affects infants, young children and the elderly.

Meningococcal meningitis is caused by the meningococcus bacterium, Neisseria meningitidis. There are five main groups that commonly cause meningococcal meningitis A, B, C, W135 and Y. Approximately 10% of the population have healthy carriage of the meningococcal bacteria in the back of their throats at any given time. From an upper respiratory infection, the bacterium can enter the bloodstream and infect the meninges and may also cause septicaemia. This infection is highly contagious. Meningococcal meningitis affects mainly young children, teenagers and young adults, and may cause local epidemics in college dormitories, boarding schools and military bases. 

Hib meningitis is caused by the Haemophilus influenzae type b (Hib) bacterium. Before 1992, Hib meningitis was the leading cause of bacterial meningitis in children. However Hib vaccines now available as part of the routine childhood immunization schedule in Canada, have greatly reduced the number of cases of this type of meningitis. When it occurs, it tends to follow an upper respiratory infection, ear infection (otitis media) or sinusitis.

Listeria meningitis is caused by the bacterium Listeria monocytogenes. This bacterium can be found in soil, in contaminated foods and animals may also have carriage. Most individuals exposed to listeria do not become ill, however pregnant women, newborns and the elderly tend to be more susceptible to infection. Listeria can cross the placental barrier, and infections in late pregnancy may cause a baby to be stillborn or die shortly after birth. People with weakened immune systems are most vulnerable. 

TB meningitis is caused by the Mycobacterium tuberculosis bacterium. A TB infection usually begins in the lungs, but the bacteria may travel through the bloodstream to the meninges and cause TB meningitis. 

Viral meningitis causes a greater number of cases of meningitis than do bacteria. There are many different viruses that can cause viral meningitis, however, enteroviruses are the most common group. Viral meningitis is usually mild and often clears on its own within two weeks.

Fungal meningitis is very rare. It typically occurs when the immune system is deficient due to disease, e.g. HIV infection, or by drug therapy. Fungal meningitis usually follows a slow progression and is often difficult to diagnose and treat. Types of fungi that cause meningitis include Cryptococcus neoformans and Candida albicans.