The World Health Organization aims to attain the highest level of health for all peoples. Its objectives include promoting complete physical, mental and social well-being without discrimination; ensuring health is valued for attaining peace and security; and extending medical knowledge to all. Membership is open to all countries, who contribute yearly and are entitled to WHO services and aid. WHO works on disease prevention and control, health systems development, research, statistics, and environmental health. It is headed by the Director-General and has regional offices around the world.