Final answer:
The most common causes of pneumonia in children aged 4 months to 5 years old is Haemophilus influenzae, whereas for children older than 5 years old, it is Streptococcus pneumoniae. Viruses can also cause pneumonia in children and the elderly.
Step-by-step explanation:
The most common cause of pneumonia in children aged 4 months to 5 years old is Haemophilus influenzae. This bacterium can be found in the throats of healthy individuals and can cause meningitis in children younger than 5 years old. However, with the use of the effective Hib vaccine, cases of H. influenzae-related meningitis have become uncommon in most countries.
For children older than 5 years old, the most common cause of pneumonia is Streptococcus pneumoniae. This bacterium is also associated with meningitis in humans aged 2 months through adulthood.
Viral pneumonia, caused by viruses like adenoviruses, influenza viruses, parainfluenza viruses, and respiratory syncytial viruses, is another common cause of pneumonia in children and the elderly.