Pulmonary surfactant is a mixture of lipids and proteins lining the lungs and controlling the surface tension of its membranes. It prevents the inner lung surfaces sticking together and stops fluid filling the lungs. Without surfactant, it would be impossible to inflate the lungs. Influenza and other infections can result in mild damage to the surfactant system allowing fluid into the lungs, causing congestion. Severe damage to the surfactant system is a life-threatening condition, called respiratory distress syndrome (RDS). Possible causes of RDS include inhaling toxic gases (eg smoke or chlorine), overdose or narcotic drugs (eg heroin or morphine), and infectious diseases. Many deaths from influenza, tuberculosis and pneumonia occur because the victim's surfactant layer is destroyed and they cannot breathe. Premature babies suffer RDS for a different reason. The surfactant system forms late in pregnancy and for babies born before it has fully developed, the effort of expanding the lungs is more more than the baby can manage. Treatment for RDS is to spray artificial surfactant into the patient's lungs.