H. influenzae is a gram-negative, coccobacillus. It was found in the respiratory tract of asymptomatic individuals. H. influenzae is the common pathogen causing respiratory tract infections, including sinusitis, otitis media, and pneumonia. It could also be highly invasive, causing meningitis and bloodstream infections. H. influenzae could be characterized by the presence of its capsule; unencapsulated strains and the encapsulated strains. Encapsulated strains could be further grouped based on 6 capsular antigens. Among them, serotype b (Hib) is the most important. It is an obligate human pathogen and an important cause of invasive bacterial infections in both children and adults, with the highest incidence among young children. However, with the introduction of vaccine, the infection rate has been declining dramatically.