Bill Gates, co-chair of the Bill & Melinda Gates Foundation, speaks during a discussion on innovation hosted by Reuters in Washington

Bill Gates, co-chair of the Bill & Melinda Gates Foundation, speaks during a discussion on innovation hosted by Reuters in Washington, U.S., April 18, 2016. REUTERS/Joshua Roberts - RTX2AJJ3

Bill Gates, co-chair of the Bill & Melinda Gates Foundation, speaks during a discussion on innovation hosted by Reuters in Washington, U.S., April 18, 2016. REUTERS/Joshua Roberts – RTX2AJJ3