Which virus can cause severe respiratory illness in infants?

Respiratory syncytial virus (RSV) is widely recognized as a primary cause of severe respiratory illness in infants and young children. This virus is highly contagious and tends to circulate during the fall and winter months. RSV leads to infections in the respiratory tract and can cause conditions ranging from mild cold-like symptoms to severe bronchiolitis and pneumonia.

Infants, particularly those who are premature or have underlying health conditions, are especially at risk for severe outcomes. The mechanism includes the virus's ability to infect the epithelial cells lining the respiratory tract, leading to inflammation, increased mucus production, and airway obstruction.

Other viruses mentioned have different disease associations. Coxsackie A virus primarily causes herpangina and hand-foot-and-mouth disease, while the measles virus is more associated with systemic illnesses including rash and other complications. Hepatitis A virus primarily affects the liver, causing symptoms related to liver function rather than respiratory illness. Thus, RSV stands out as the correct answer due to its specific link to serious respiratory issues in infants.