A 2-year-old child was admitted to the hospital with acute meningitis. The Gram stain revealed Gram-positive short rods, and the mother indicated that the child had received "all" of the meningitis vaccinations. What is the most likely cause of the disease?

Except during a meningococcal epidemic, H. influenzae is the most common cause of bacterial meningitis in children. The organism is occasionally found to be associated with respiratory tract infections or otitis media. H. influenzae, N. meningitidis, S. pneumoniae, and Listeria account for 80 to 90% of all cases of bacterial meningitis. A purified polysaccharide vaccine conjugated to protein for H. influenzae type B is available. A tetravalent vaccine is available for N. meningitidis and a 23-serotype vaccine for S. pneumoniae. No vaccine is available for Listeria.