The World Health Organization (WHO) is a specialized agency under the United Nations and the largest public health organization in the world, with its headquarters in Geneva, Switzerland. The purpose of the World Health Organization is to enable people all over the world to achieve the highest possible health level. The organization defines health as "the perfect state of physical, mental and social life". The main functions of the World Health Organization include: promoting the prevention and treatment of epidemic and endemic diseases; To provide and improve teaching and training in public health, disease treatment and related affairs; Promote the determination of international standards for biological products. As of May 2005, WHO has 193 member countries. The current Director-General is Hong Kong resident Margaret Chan.