It was isolated by Pfeiter from the nasopharyngeal secretions of an influenza patient in 1892. Most cases of influenza bacillus meningitis are caused by influenza bacillus type b. Influenza bacillus generally exists in the upper respiratory tract of humans. According to the different components of the capsular polysaccharide antigens, the bacterium is divided into six serotypes, namely a, b, c, d, e and f, using type-specific immune serum for capsule swelling tests. Among them, type b bacteria are the most pathogenic to humans (infants and young children). Influenza bacillus only invades humans, and the age of onset is mainly infants from 3 months to 3 years old. Because infants under 2 months old have bactericidal antibodies from their mothers, they rarely get sick. Later, as they age, the specific bactericidal antibodies from their mothers decrease, and they themselves cannot produce enough antibodies, so they are prone to this disease. Children over 5 years old and adults rarely get sick. If the disease occurs after the age of 5, attention should be paid to check for anatomical and immune deficiencies. This disease can occur throughout the year, but it is most common in autumn and winter. There have been many reports of two children in the same family at the same time.