Today is World Malaria Day. The World Health Assembly established World Malaria Day in 2007 to commemorate global efforts to control malaria. The World Health Organization says about 3.3 billion people in 2009 – half of the world's population – were at a risk of contracting malaria. WHO says, people living in the poorest countries are most vulnerable. There are about 250 million malaria cases every year. It is the biggest killer of children under the age of five in sub-Saharan Africa.