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 experts have a research gift from an unlikely benefactor: El Nino. Combined with an underlying global warming trend, say scientists who track such things, El Nino will probably make 1998 the warmest year in several centuries.