More than 350 million people suffer from depression globally, said the World Health Organisation, an announcement timing with World Mental Health Day today.
"It is not a disease of developed countries, it is a global phenomenon. It's present in both genders and in rich and poor populations," Dr Shekhar Saxena, head of the WHO's mental health and substance abuse department, told reporters in Geneva.
No region is free from the disorder and around 5 per cent of the world's population suffers depression in the course of a year, the health expert said.
Fifty percent more women suffer symptoms than men, said Dr Saxena, who added that post-natal depression affected one in five mothers and one in 10 of all young mothers in the developed world.
According to the UN's global health arm, depression is more than just a bout of the blues, rather a "sustained feeling of sadness for two weeks or more" which interferes with "work, school or home".
At its worst, depression can lead to suicide, the WHO expert said, citing a "very clear correlation".
Nearly one million people take their lives every year and more than half of them have depression, Dr Saxena said, although he noted that it was not the sole cause.
"Depression has existed for centuries, the news is, we're not doing anything about it," he said, noting that the shame associated with having the illness meant that fewer than half of those with depression received the care they need.
The figure dropped to less than 10 per cent in many countries, he added.
Effective treatment was available, but health workers needed to do more to spot the signs of depression in people who complained of other symptoms, particularly in children as young as 12 and young adults who they did not expect to have the illness.