Which bacterium is the main cause of bacterial meningitis in many young children before vaccination?

Haemophilus influenzae was a major cause of bacterial meningitis in young children before vaccination.

H. influenzae is a small Gram-negative bacterium that can live in the upper respiratory tract. The encapsulated type b strain, usually called Hib, was historically responsible for many cases of meningitis, epiglottitis, and bloodstream infection in children.

The Hib conjugate vaccine transformed childhood health by sharply reducing invasive Hib disease. The species name is misleading: H. influenzae does not cause influenza, which is a viral respiratory illness caused by influenza viruses. The bacterium was named during an early, incorrect attempt to connect it with influenza.

Non-typeable strains lack the prominent capsule of Hib and commonly cause ear and respiratory infections. Other bacteria, especially Streptococcus pneumoniae and Neisseria meningitidis, remain important causes of bacterial meningitis today.