"The global population is growing, but the number of health workers is stagnating or even falling in many of the places where they are needed most," said Lee Jong-wook, director-general of the World Health Organization.
Doctors and nurses are urgently needed in the 57 worst-affected countries to immunize children against diseases and to treat HIV/AIDS, malaria and tuberculosis, said the 209-page annual World Health Report.
"The global shortage approaches 4.3 million health workers," the report said, adding that the lack is greatest in the areas that need medical care the most — South and East Asia and sub-Saharan Africa.
"Life expectancies have collapsed in some of the poorest countries to half the level of the richest — attributable to the ravages of HIV/AIDS in parts of sub-Saharan Africa" and to civil war and other strife in more than a dozen countries, it said.
At the same time there are "growing fears, in rich and poor countries alike, of new infectious threats such as SARS and avian influenza," the report said.
SARS, or severe acute respiratory syndrome, showed in 2003 how quickly a new disease could spread in reaching 30 countries. There were 8,000 cases of the disease, about 800 of them fatal.
Health experts fear a similar spread if bird flu should mutate into a strain of virus that easily passes from person to person, setting off a flu pandemic that could kill millions.
"Recent concern about the threat of avian influenza has drawn attention to the devastating impact a global pandemic could have, given the current shortage of health workers, combined with their insufficient preparedness and often poor working conditions," WHO said. "Sudden catastrophic events can quickly overwhelm local and national health systems already suffering from staff shortages or lack of funds."
The burden falls on every country — even the wealthy — to increase the number of health workers, it said, noting that the richest countries are filling their shortages through a "brain drain" pulling doctors from the poorest countries.
It said one of every four doctors trained in Africa is working in the 30 mainly industrialized nations in the Organization for Economic Cooperation and Development. The rate for African nurses is one in 20 working in the OECD.
"Such countries are likely to attract even more foreign staff because of their aging populations who will need more long-term, chronic care," said WHO Assistant Director-General Timothy Evans.
The report said that 16.5 percent of the population of the United States was over 60 as of 2004, up from 16.3 percent a decade earlier.
The percentages in some European countries is even higher and growing faster, but many countries in Africa have between 4 and 5 percent over 60.
Some countries take advantage of the brain drain by receiving money sent home by the health workers abroad, the study said.
"The Philippines has been training nurses for export for many years," with many of them going to the United States. In 2004 they and other Filipinos living abroad sent home US$8.5 billion.
Cuba also has exported thousands of health workers as part of bilateral relations with other countries.
The report said more than a billion people worldwide lack access to the most basic health care — often because there is no health worker.
Better access to health care could prevent many of the 10 million deaths a year from infectious diseases and complications in pregnancy and child birth, it said.