Haemophilus influenzae (H. influenzae) is a Gram-negative coccobacillus which was first identified in 1892. Some strains of H. influenzae have a polysaccharide (carbohydrate) capsule (outer covering). The capsule makes it very hard for the immune system to defend the body against H. influenzae. Encapsulated strains of H. influenzae cause serious illnesses such as pneumonia, meningitis, bacteraemia, epiglottitis, osteomyelitis, septic arthritis, septicaemia, cellulitis and pericarditis. A particular strain, known as H. influenza serotype b (Hib) is responsible for most cases of serious illness due H. influenzae. There are six serotypes named a-f. Strains of H. influenzae that do not have a capsule tend to cause non-serious disease such as middle ear infections (otitis media), worsening of asthma or chronic lung disease (chronic obstructive pulmonary disease), community-acquired pneumonia, sinusitis and neonatal and maternal sepsis.