Table 1.

Factors involved in the lung response to respiratory syncytial virus infections.

Viral factors	Host and environmental factors	Immunological factors	Nonimmunological factors
(i) Subgroup [28] (ii) Viral load and high infectivity [4] (iii) Immune evasion mediated by G protein [28–32] (iv) NS1 and NS2 proteins [14]	(i) Preexisting diseases [34, 35] (ii) Young age [34, 35] (iii) Exposure to tobacco smoke and traffic-derived pollution [34, 35] (iv) Lack of breastfeeding [36] (v) Daycare attendance [34, 35] (vi) Siblings [34, 35] (vii) Meteorological factors [37, 38] (viii) Low levels of vitamin D [39] (ix) Immunodeficiency [40]	(i) Innate immunity (neutrophils, eosinophils, and complement) [41–46] (ii) Adaptive immunity (IgA, IgG, and T lymphocytes) [47–49] (iii) Th1/Th2 balance [44]	(i) Anatomy of the airways in young infants [50] (ii) Surfactant proteins [51]