FIG. 1.

Interplay of factors that cause disease following respiratory viral infection and impact of infancy. The ultimate cause of illness/disease following respiratory viral infection is airway occlusion, which leads to a reduction in gaseous exchange, leading to respiratory distress. This airway occlusion can be either immune or virally mediated and most probably is a combination of both. Early life has an effect on both virus- and immune-mediated damage. The infant immune system is skewed to a hyporesponsive phenotype, with a reduced type I interferon response leading to a higher viral load. The adaptive immune response is also skewed and limited in its effect. These factors act in combination with small body size and small airways to further increase disease severity.