Bill and Melinda Gates in Mapinga, Tanzania, in 2011.
Bill Gates during a site visit to the ACOS Ethiopia processing facility in 2012.
A young boy samples an orange-fleshed sweet potato in a busy market in Mwasonge, Tanzania.
House-to-house polio vaccination team members Heena Khatun and Rukhsar Khatun work in the Kamla Nehru Nagar slum in Patna, India, in December 2010.
An ASHA worker visits the Chandan slum to discuss general health concerns and family planning practices with residents in Uttar Pradesh, India, on June 4, 2014.
Melinda Gates meets with a local women's group to discuss sanitation issues in New Delhi in September 2014.