Despite popular concerns about the threats to public health posed by communicable diseases, cancer, heart disease, stroke, and mental illness will emerge as the world's biggest killers by the year 2025, says a major study of global health trends issued here today.

And most health systems will be unprepared to shoulder the burden, according to the same report. As life expectancy rises, especially in developing countries, the incidence of diseases associated with aging will go up as well, straining the resources of many developing countries, researchers forecast. By 2020, non-communicable illnesses will rise to 73 percent of all deaths, up from 55.8 percent in 1990, according to the findings of the study directed by the World Health Organization and sponsored by the governments of Australia, Canada, Britain, Norway, Sweden, and Switzerland. The report, whose other sponsors include U.S. foundations and the World Bank, says the new trend will be bolstered by two main developments: The proportion of the population aged 45 and over in 2020 is expected to rise 200 percent higher than that of 1990; and tobacco-related deaths could triple to 8.4 million a year within 25 years.

Researchers say tobacco-related health problems are growing more rapidly than the HIV epidemic, and that this problem could cause more deaths than any other single ailment. "Non-communicable disease will be the coming epidemic in low-income and middle-income countries," says Professor Christopher Murray of the Harvard School of Public Health. Only in sub-Saharan Africa will infectious diseases kill more than non-communicable diseases in the next 25 years, he says.

By 2020, heart disease will be the leading culprit in the "total disease burden" -- calculated as the years subtracted from a healthy life by disability or premature death. Depression will be the second largest problem worldwide, but it will be number one in developing countries. Road accidents will the third largest cause of lost healthy years. Other causes will include lower respiratory infections, tuberculosis, war, diarrheal diseases, and AIDS-causing HIV infections.

The report forecasts that the incidence of infectious diseases will decrease in developing countries, even though cases of tuberculosis and AIDS will continue to rise. TB is likely to account for at least 2.3 to 3.3 million deaths a year by 2020, while AIDS could kill up to 1.7 million people a year. Sponsors of the study say the new research is key in that it outlines the first road map for governments and health care providers on requirements for medical research and development. The findings of the study are published in two reports.

The first, "The Global Burden of Disease and Injury Series," assesses deaths as well as disability, using the "disability-adjusted life year" (DALY). The measure combines years of life lost through premature death with years lived with a disability to assess the loss of healthy life years. Psychiatric and neurological illnesses, particularly depression, alcoholic dependence, manic depression, and schizophrenia, are expected to increase their share of disease burden around the world to almost 15 percent by 2020 -- up from 10 percent today. Researchers say only about one percent of all deaths are now caused by these diseases, and consequently, their importance has been overlooked.

Cardiovascular disease will account for 15 percent of all global disease burden by 2020. Cancers, led by lung cancer, and increased respiratory disease a expected to double to almost 10 percent of total disease burden. As the adult proportion of the population swells, so do the absolute numbers of cases of chronic diseases experienced by adults. Fertility has declined sharply in recent decades, causing a rise in the adult proportion of the population relative to children. By 2020, the number of middle-aged adults will have doubled, whereas the population under five will grow by only 20 percent. In India, the burden of non-communicable diseases is expected to almost double from 29 percent in 1990 to 57 percent in 2020. The findings of the new study have been published in a second report, titled, "Investing In Health Research and Development," which assesses the current and future health needs of each region. It uncovers "a severe mismatch" between the huge burden of diseases currently caused by conditions such as diarrheal disease, tobacco-related disease, and pneumonia, and the meager resources available for research to develop tools for their control.

The Ad Hoc Committee on Health Research Relating to Future Intervention Options -- a gathering of health researchers and policymakers from all continents -- has identified four major health threats facing governments and health systems.

They must deal with the traditional enemies to maternal and child health; they must defend their populations against a continually changing threat from microbes, including tuberculosis, pneumococcus, malaria, HIV/AIDS and other sexually transmitted diseases by developing such affordable tools as a malaria vaccine, an HIV vaccine, and more effective methods to deliver TB drugs.

The committee also recommends that governments respond to chronic diseases and injuries that are unfolding in most developing regions both by improving basic data on the scale of the problems and by developing locally relevant and cost-effective solutions to them. Then they must improve health services by learning from other countries which approaches to health care work best. Most health systems are not prepared for the future burdens, researchers say, noting that access to treatment is minimal and cost prohibitive, particularly in developing countries.

"Right now, the allocation of funds for (research and development) is often irrational and based on inadequate information," says Tore Godal, director of the U.N. Development Program/World Bank/WHO Special Program for Research and Training in Tropical Diseases. "Some of the gravest health problems receive only pitiful resources, while comparatively small problems receive large shares," says Godal, one of the study's directors.