Depression is the leading cause of disability
Depression is the leading cause of disability as measured by YLDs and the 4th leading contributor to the global burden of disease (DALYs) [Disability Adjusted Life Years) in 2000. By the year 2020, depression is projected to reach 2nd place of the ranking of DALYs calcuated for all ages, both sexes. Today, depression is already the 2nd cause of DALYs in the age category 15-44 years for both sexes combined [DALY represents the loss of one year of equivalent full health.]
WHO, Mental Health, What is Depression accessed 8.08 http://www.who.int/mental_health/management/depression/definition/en