Haemophilus influenzae is a gram-negative bacterium commonly found in the human respiratory tract. It can cause infections such as meningitis, particularly in young children. The bacterium is classified into six serotypes based on its polysaccharide capsule, with type b being the most virulent and a former cause of epidemics before the development of an effective vaccine in the late 1980s. Transmission occurs through respiratory droplets, with symptoms including fever, cough, and congestion. Treatment involves antibiotics, and vaccination has dramatically reduced the incidence of invasive disease.