As many as 400 children have died of lead poisoning-related illnesses in Nigeria since March, two international aid groups say, and as many as 30,000 people could be affected by lead contamination.The deaths occurred predominantly in children under the age of 5 in the state of Zamfara, according to Lauren Cooney, the emergency manager for Medecins Sans Frontieres. The group is also known by its English name, Doctors without Borders.