Scientists have long theorized that climatic changes related to global warming could unleash outbreaks of diseases like malaria, dengue fever, cholera and heat stroke. But with the modest amount of warming experienced so far, they have been unable to produce much hard evidence.

Now the effects of this year's El Nino will probably help make 1998 the warmest year in several centuries, and the resulting heavy rains, droughts and other extremes are precisely what many scientists expect will also result from a predicted warming of the globe in the decades ahead.So in the belief that 1997 and 1998 may provide a taste of what is to come, those who pay attention to the health effects of climate are now pulling together data about weather-related illness and death in recent months. While most of the evidence is circumstantial, it suggests that amid all the scientific smoke there is some fire.

These are some of the clues:

- The World Health Organization reports "quantitative leaps" in the incidence of malaria around the world, coincident with extreme weather events associated with El Nino. Both heat and variations in rainfall affect transmission of the disease by mosquitoes.

- Tens of thousands of people in Kenya and Somalia were afflicted by another mosquito-borne disease, Rift Valley fever, and at least 200 died, after the heaviest rains since 1961, attributed to El Nino, fell on the region.

- The incidence of cholera increased markedly over the last year in Latin America, where an epidemic had already been in progress for seven years, and in parts of Africa. In both cases the surge in cases was associated with heavy rainfall and floods linked to El Nino.

- In the southern Rockies, a warm, wet winter brought on by El Nino has produced abundant food and cover for deer mice, which transmit the deadly hantavirus to humans. The number of mice has increased, and the deaths of three people have been ascribed to the disease. Federal disease-control agents are still analyzing the problem.

- Forest fires in Southeast Asia, made possible by a killing drought attributed to El Nino, subjected hundreds of thousands of people to respiratory ailments.