Indian schoolchildren write English alphabets on slates at a primary school outside Hyderabad in June. India is on track to overtake China as the most populous nation in just 16 years. Noah Seelam/AFP/Getty Images hide caption

itoggle caption Noah Seelam/AFP/Getty Images

Dr. Sandrine Courtial examines 3.5-month-old Ayline Alhas at the Centre de Protection Maternelle et Infantile (PMI) in Savigny-sur-Orge, France, before giving her a vaccination. Ayline's mother, Melissa, has been bringing her in for free well-child check-ups since her birth. Olivier Pascaud for NPR hide caption

itoggle caption Olivier Pascaud for NPR

Levi Aron, left, is arraigned in Brooklyn criminal court Thursday, July 14, 2011, in New York. Aron, 35, is charged with luring 8-year-old Leiby Kletzky to his home on Monday, and then smothering him and chopping him up when he learned that a search was under way for the missing child. Detectives found the boy's feet in Aron's freezer. Louis Lanzano/AP hide caption

itoggle caption Louis Lanzano/AP

In the U.S., children typically receive vaccines for diseases such as mumps, measles, rubella and tetanus. Dmitry Naumov/iStockphoto.com hide caption

itoggle caption Dmitry Naumov/iStockphoto.com