(Ivanhoe Newswire) -- Extreme prematurity is known to increase the risk of a respiratory syncytial virus infection (RSV), but a new study shows that even mild prematurity can increase the odds.

RSV infection is the leading cause of lower respiratory tract infections in infants and often leads to pneumonia. Babies exposed to supplemental oxygen or assisted ventilation during the neonatal period are those at greatest risk.

The study, conducted by the Kaiser Permanente Division of Research, included 108,794 babies of at least 33 weeks gestation. Babies born at 34 to 36 weeks had 70 percent increased odds of RSV infection compared to the babies 38 to 40 weeks. Even at 37 weeks, the babies had 37 percent increased odds of infection.