Final answer:
Haemophilus influenzae Type b (Hib) causes meningitis, particularly in children between 2 months and 5 years old, but this has become uncommon due to the Hib vaccine. Nonencapsulated strains of H. influenzae are more associated with respiratory infections such as pneumonia and otitis media.
Step-by-step explanation:
The form of Haemophilus influenzae that causes meningitis is known as Haemophilus influenzae Type b (Hib). This encapsulated pleomorphic gram-negative coccobacillus was once a primary cause of meningitis in children aged 2 months through 5 years. However, due to the effective Hib vaccine, the incidences of Hib meningitis have significantly decreased. Most children become immune to this bacterium by the age of five, but infants older than 2 months are still susceptible to serious diseases caused by Hib until they can develop a sufficient antibody response.
Conversely, the nonencapsulated strains of H. influenzae are more associated with causing respiratory infections such as Haemophilus pneumonia. These strains primarily affect the elderly and are capable of causing infections such as otitis media, commonly known as ear infections. The bacteria are spread by droplets from coughs and only grow in media that provide specific growth factors.