Vaccines work by exposing the immune system to molecules similar to those found in viruses and bacteria, training it to recognize and fight off future infections. As a result of vaccination programs led by WHO and UNICEF, global immunization rates have reached their highest levels ever, protecting millions of children from deadly diseases annually. While vaccines have essentially eradicated smallpox and brought many other illnesses under control, continued vaccination efforts are needed to defend against pneumonia, diarrhea, hepatitis and other leading causes of childhood mortality worldwide.