In recent years scientists have become increasingly concerned about the long-term effects of deteriorating environmental conditions on the health not only of humans but also of nature itself. "We are no longer talking only of an increased exposure to specific extraneous hazards as a cause of bad health. We are also recognizing the depletion or disruption of natural biophysical processes that are the basic source of sustained good health," points out epidemiologist Tony McMichael of the London School of Hygiene and Tropical Medicine (152).

At increasing risk, according to McMichael, are the ecosystems that determine food productivity and such global systems as the hydrological cycle—in which water evaporates from bodies of water and returns to them after falling as precipitation—and the stratospheric "ozone shield" that protects against excessive solar ultraviolet radiation. Such environmental changes would have a wide variety of negative effects on human health (see Table 2).

Some ecologists use the term "environmental distress syndrome" to identify deteriorating environmental conditions and resulting threats to health. Paul Epstein of Harvard Medical School lists four symptoms of this syndrome (61):

• Re-emerging infectious diseases, including typhoid, cholera, and pneumonia, and the emergence of new diseases, such as drug-resistant tuberculosis and human reproductive disorders linked to industrial chemicals.
• Loss of biodiversity and the consequent loss of potential sources of new medicinal drugs and food crops.
• The decline in pollinators, such as bees, birds, bats, butterflies, and beetles, which are indispensable to the reproduction of flowering plants.
• Proliferation of harmful algae along the world's coastlines, leading to more deadly outbreaks of diseases such as ciguatera poisoning and paralytic shellfish poisoning.

Such symptoms raise a disturbing question: At what point might the depletion of the world's ecological and biophysical capital undermine global public health? For instance, WHO has reported that a recent epidemic of meningitis in sub-Saharan Africa could be linked to an expansion of degraded agricultural and grazing land-a result of changes in land—use patterns (less vegetative cover) and less rainfall due to regional climate change triggered by human activities (152).

One study links a marked increase in diarrheal diseases among Peruvian children to the frequent and severe El Niño weather patterns, which are characterized by heavy rainfall and result from unusual warming of the equatorial Pacific Ocean. El Niño has been linked to outbreaks of dengue fever, malaria, and cholera. Its patterns have been made worse by global climate change (33) (see side-bar, Global Warming: Worrisom Signs).

The organisms that transmit such diseases as malaria, dengue fever, and schistosomiasis are sensitive to temperature, humidity, rainfall patterns, and wind. Increases in temperature tend to accelerate the life cycles and decrease the incubation periods of the parasite or virus. These changes extend the time during which the diseases are transmitted and encourage their spread to new areas (62 , 131, 150).

Global climate change also has indirect effects upon the transmission of diseases (150). For example, global warming would increase the need for irrigation. In hot climates the prevalence of schistosomiasis already has increased due largely to the expansion of irrigation systems and dams. These systems support more water snails, an intermediate host of the schistosomiasis worm, and bring more people into closer contact with worms (131).