Final answer:
The common cause of pneumonia in young children after viral illnesses like influenza or chickenpox is a bacterial infection, usually by pathogens like Streptococcus pneumoniae or Haemophilus influenzae.
Step-by-step explanation:
A common cause of pneumonia in children younger than 5 years of age after infection with influenza or chicken pox is a bacterial infection. While pneumonia can be caused by a variety of pathogens including viruses, fungi, and parasites, the vast majority of pneumonia cases are bacterial in origin, particularly following a viral illness such as influenza. Important bacterial pathogens include Streptococcus pneumoniae, Haemophilus influenzae, and Mycoplasma pneumoniae. These bacteria can cause pneumonia by inflaming the alveoli and flooding them with fluid, which makes breathing difficult and impairs gas exchange in the lungs.