What is the most common cause of bronchiolitis and pneumonia in infants under one year of age?

Respiratory Syncytial Virus, commonly known as RSV, is recognized as the most prevalent viral cause of bronchiolitis and pneumonia in infants under one year of age. This virus leads to inflammation and congestion in the small airways of the lungs, making it particularly concerning for this age group, as their immune systems are still developing and they have smaller airways, which can exacerbate respiratory issues.

RSV typically circulates in seasonal outbreaks and infects nearly all children by the age of two. It can cause a range of illnesses, from mild upper respiratory tract infections to more severe lower respiratory conditions like bronchiolitis and pneumonia, especially in infants or children with preexisting health conditions. The unique vulnerability of infants to RSV is attributed to their naive immune systems, smaller lung capacity, and rapid breathing patterns.

Other viruses and bacteria can cause respiratory infections, but RSV is specific in its frequency and impact on this demographic, making it the leading cause of these conditions in young children. This understanding is critical for prevention strategies, as RSV can lead to substantial morbidity in this population.