The vast majority of newborns enter the world healthy. But sometimes, infants develop conditions that require medical tests and treatment. Newborns are particularly susceptible to certain diseases, much more so than older children and adults. Their new immune systems aren't adequately developed to fight the bacteria, viruses, and parasites that cause these infections. As a result, when newborns get sick, they may need to spend time in the hospital — or even the neonatal intensive care unit (NICU) — to recover. Although it can be frightening to see your baby hospitalized, a hospital stay is often the best way back to good health for a sick newborn. There is a higher risk of infection for preterm or low birth weight neonates. Infant respiratory distress syndrome is often a condition of preterm neonates that can have long-term negative consequences; it can also arise following an infection. In some instances, neonatal respiratory tract diseases may increase the susceptibility to future respiratory infections and inflammatory responses related to lung disease.