Haemophilus influenzae (H. influenzae) is a type of bacteria that can cause various infections, ranging from mild to severe. It is commonly associated with respiratory tract infections, such as pneumonia and bronchitis, but can also cause invasive diseases like meningitis and septicemia. There are six types of H. influenzae (a-f), with type b (Hib) being the most virulent and responsible for the majority of severe infections.