on which local rodents feed became more abundant. The result, when combined with the drop in predators, was a tenfold increase in rats and mice (Levins et al., 1993; Epstein, 1994) and the emergence of a "new" diseasecalled hantavirus pulmonary syndromestemming from a virus and transmitted through rodent droppings.
The effects of climatic variations on ecosystems have been shown to be related to outbreaks of malaria (Bouma et al., 1994a, 1994b; Hales et al., 1996), dengue fever, and other mosquito-borne diseases (Loevinsohn; 1994), which spread when appropriate rainfall conditions or higher daytime minimum temperatures favor mosquito breeding and survival. Climate variations, by altering functional relationships within the marine food web (Roemmich and McGowan; 1995), may increase the risks to humans from paralytic, diarrheal, neurologic, and amnesic shellfish poisoning (Epstein et al., 1993b) and cholera (Colwell, 1996). It is at least suggestive that domoic acid poisonings, resulting from diatom blooms that produce toxins in seafood, appeared in Canada in the El Niño year of 1987 (Todd, 1989; Todd and Holmes, 1993), and related phenomena occurred in California, Argentina, and Scandinavia in the El Niño year of 1992 (Ludlohm and Skov, 1993; Carreto and Benevides, 1993). The cold phase of ENSO can also create conditions, such as intense rains and flooding following prolonged drought, that are optimal for breeding insect vectors of dengue fever and Venezuelan equine encephalitis and for rodent transmission of leptospirosis (Epstein et al., 1995). Many such associations have been documented, and where the ENSO signal is closely correlated with weather patterns, predictive models of conditions conducive to disease outbreaks may be useful. The "ENSO Experiment" begun in spring 1997 by the National Oceanic and Atmospheric Administration's Office of Global Programs coordinates scientific work by health researchers, ecologists, and meteorologists examining the relationships between ENSO and a variety of infectious diseases and marine ecological disturbances.
Human coping with disease has primarily involved year-round precautions such as individual maintenance of good nutrition, food refrigeration, and collective programs of sewage treatment, water chlorination, testing for red tide and fecal coliform bacteria, air quality testing and alerts, mass vaccination, and the like. Some coping activities also involve seasonal routines, such as the use of mosquito netting and insect repellents and alerts for heat waves and extreme cold. Public health systems do, however, also respond to forecasts of disease outbreaks, for example, with annual programs to develop and disseminate vaccinations against the influenza strains considered most likely to infect a population in a given winter. Thus, public health is a potential beneficiary of improved climate forecasting.