Definition, Etiology, and Incidence
Meningitis is an infection of the membranes (called meninges) that surround the brain and spinal cord. The most common causes of meningitis are viral infections that usually resolve without treatment. However, bacterial infections of the meninges are extremely serious illnesses, and they may result in death or brain damage even if treated. Fungi, chemical irritation, drug allergies, and tumors can also cause meningitis.
Acute bacterial meningitis is a true medical emergency that requires immediate hospital-based treatment. Bacterial strains that cause meningitis include Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis (meningococcus), Listeria monocytogenes, and many others. About 17,500 cases of bacterial meningitis occur each year in the United States.
Viral meningitis is milder and occurs more often than bacterial meningitis. It usually develops in the late summer and early fall, and it often affects children and adults who are less than 30 years old. Seventy percent of the infections occur in children who are less than 5 years old. Most viral meningitis is associated with enteroviruses, which are viruses that commonly cause stomach flu. However, many other types of viruses can also cause meningitis; for example, viral meningitis may occur as a complication in people with genital herpes.
Some forms are bacterial meningitis are contagious. The bacteria are spread through the exchange of respiratory and throat secretions. None of the bacteria that cause meningitis are as contagious as the common cold or flu, and they are not spread by casual contact or by simply breathing the air where a person with meningitis has been.
Sometimes the bacteria that cause meningitis have spread to other people who have had close or prolonged contact with a patient with meningitis caused by Neisseria meningitidis (meningococcal meningitis) or Haemophilus influenzae type B (Hib). People in the same household or day-care center or anyone with direct contact with a patient's oral secretions should be considered at increased risk of acquiring the infection. People who qualify as close contacts of a person with meningitis caused by N meningitidis should receive antibiotics to prevent them from getting the disease. Antibiotics for contacts of a person with Hib meningitis disease are no longer recommended if all contacts 4 years old or younger are fully vaccinated against Hib disease.
Signs and Symptoms
High fever, headache, and neck stiffness are common symptoms of meningitis in anyone over the age of 2 years. These symptoms can develop over several hours, or they may take 1 to 2 days to fully develop. Other symptoms may include nausea, vomiting, sensitivity to light, confusion, and sleepiness. In newborns and small infants, the classic symptoms of fever, headache, and neck stiffness may be absent or difficult to detect. The infant may appear slow or inactive, be irritable, have poor feeding, or be vomiting. As the disease progresses, patients of any age may have seizures.
Diagnosis
Early diagnosis is very important. If symptoms occur, affected individuals should seek treatment immediately. Diagnostic tests include the following:
lumbar puncture with cerebrospinal fluid (CSF) glucose measurement and CSF cell count; 
gram-stain and culture of CSF; 
chest x-ray to look for other sites of infection; and 
a computed tomography scan of the head to look for hydrocephalus, abscess, or deep swelling.