Haemophilus influenzae is a type of bacteria that can cause various infections, particularly in young children and individuals with weakened immune systems. It is a small, gram-negative organism that requires specific nutrients to grow, which is why it is often found in the human respiratory tract. There are several strains of Haemophilus influenzae, with type b (Hib) being the most notable for causing serious diseases such as meningitis and pneumonia. Vaccination against Hib has significantly reduced the incidence of these infections, making it an important public health achievement.