Pneumonia Caused By Haemophilus influenzae H. influenzae, named because it was erroneously implicated as a cause of influenza during the pandemic of 1889, is a relatively common cause of bacterial pneumonia, second only to Streptococcus pneumoniae in most studies of community-acquired lung infections. Strains containing the type b (Hib) polysaccharide capsule are most virulent and most likely to cause serious disease, including meningitis, epiglottitis, and bacteremic pneumonia. These infections have nearly disappeared in the USA and other developed countries because of use of Hib vaccine. Strains of H. influenzae that commonly colonize the upper airways of adults are usually nonencapsulated (not type b). These strains may colonize the lower respiratory tract of patients with chronic bronchitis and are often implicated in exacerbations of chronic bronchitis. Symptoms, Signs, and Diagnosis Hib pneumonia usually occurs in children (median age: 1 yr). Coryza precedes most cases, and early pleural effusions occur in about 50%. Bacteremia and empyema are not common. Most adults have infections involving nonencapsulated strains with a bronchopneumonia that resembles other bacterial pneumonias. Gram stain of expectorated sputum shows numerous, small, gram-negative coccobacilli; the organism is relatively fastidious and frequently colonizes the upper airways so that false-negative and false-positive cultures are common. Prophylaxis and Treatment Prophylaxis with H. influenzae type b (Hib) conjugate vaccine is advocated for all children to be given in three doses at 2, 4, and 6 mo of age. About 30% of H. influenzae strains produce -lactamase and are resistant to ampicillin. Thus, preferred treatment is trimethoprim-sulfamethoxazole (TMP-SMX) 8/40 mg/kg/day po or IV for children or 1 or 2 tablets of 160/800 mg bid for adults; cefuroxime 0.25 to 1 g IV q 6 h; cefaclor 40 mg/kg/day po in 3 divided doses for children or 500 mg po q 6 h for adults; or doxycycline 100 mg po bid (contraindicated in children <= 8 yr). Ampicillin 100 mg/kg/day IV in 4 divided doses (maximum, 2 to 3 g/day) for children < 20 kg or 250 mg to 1 g q 6 h for children > 20 kg and adults can be used to treat nonresistant strains. Alternative regimens are amoxicillin 20 to 40 mg/kg po tid for children < 20 kg or 250 to 500 mg po tid for children > 20 kg and adults. Fluoroquinolones and azithromycin are also active.