Final answer:
Haemophilus influenzae is the common extra meningitis bug in both old and young patients, particularly the Hib strain. It is typically found in the throats of healthy individuals, but can cause serious disease in infants older than 2 months.
Step-by-step explanation:
The common extra meningitis bug in old and young patients is Haemophilus influenzae. Specifically, the serotype b (Hib) strain of H. influenzae is responsible for causing meningitis in children aged 2 months through 5 years. This bacterium is typically found in the throats of healthy individuals, including infants and young children. While most children develop immunity to H. influenzae by the age of 5, infants older than 2 months do not produce a sufficient protective antibody response and are susceptible to serious disease.