(also spelled Haemophilus) Bacteria that are generally harmless passengers in the upper respiratory system. One strain, H. influenzae type b or Hib, is the major disease-producing species of the genus and the prime cause of meningitis in children under the age of three. In addition to bacterial meningitis, H. influenzae is responsible for other diseases, including epiglottitis, sepsis, cellulitis, pneumonia, and septic arthritis.