The Pacific water-warming phenomenon known as El Niņo has already been blamed for massive flooding, fires, and unusually warm temperatures for many regions of the world.
The warm and wet weather it brings may also mean a spread of tropical diseases.
Health specialists say the rainy winter has nourished all sorts of biological pests, from ants to bubonic plague. A week or two of 80-degree weather will push them out of hiding, said Joe Krygier, environmental health specialist for the San Bernardino County, California, vector control program.
Overgrown vegetation promised bugs of all kinds. In California, Riverside County authorities offered help clearing weeds, draining standing water and spraying for mosquitoes.
Krygier said people who live in mountain areas where bubonic plague is a perennial concern should be on guard for rats and mice. That means closing off holes into homes and keeping pet food out of reach.
The plague is transmitted by rodents' fleas. More rodents also mean more ticks, which can carry Lyme disease.
Global warming could aggravate diseases' spread
The disease threat is expected to last only as long as the effects of El Niņo do. But some public health experts say the passing threat could become a permanent problem if predictions of global warming are true.
``Clearly, the extreme events that we're now experiencing in California and Florida, and in Georgia and Alabama, and the ice storms in Maine and New Hampshire, are going to have an impact on our understanding that our climate system is becoming unstable,'' said Paul Epstein, a doctor at Harvard University Medical School.
Cold winter temperatures usually keep many tropical diseases in check. Mosquitoes can carry encephalitis, malaria and dengue fever, and field mice have been known to carry the deadly disease hantavirus. Populations of both are killed off, and kept at relatively safe levels, when temperatures drop.
But the warmer the winter, the wider these sicknesses can spread. Malaria and dengue fever are projected to reach well into the United States if the recent trend of increasing temperatures continues.
Hantavirus, a respiratory disease that killed some 60 people in the Southwest after the last big year of El Nino rains, could take an even higher death toll after this year's rainy winter.
El Niņo and the problems it causes come and go. In contrast, global warming could make the diseases a permanent fixture in the southern half of the United States.
``We are looking at an emergence of new diseases and a resurgence of old diseases, and redistribution of old diseases on a global scale,'' Epstein said.
Not all members of the public health community are buying the connection between a warming Earth and increased illness, however.
Just as there are uncertainties about how much, how soon, and whether global warming will happen, there is also hope that spreading diseases will be held in check by advances in medical treatment.