Four million health workers are needed to combat the "chronic shortage" around the world, a report from the World Health Organization warned.

Fifty-seven countries have a serious shortage of health workers, affecting children's jabs, pregnancy care and access to treatment, it said. The WHO's World Health Report 2006 said the shortage affected how diseases such as HIV/Aids could be tackled.

In recognition of the vital contribution of health workers to society and to raise awareness about their plight, WHO has devoted this year's observance of World Health Day, on 7 April, to the world's health workforce.