The ecology and transmission dynamics of infectious diseases are complex and, in at least some respects, unique for each disease within each locality. Some infectious diseases spread directly from person to person; others depend on transmission via an intermediate "vector" organism (e.g., mosquito, flea, tick), and some also may infect other species (especially mammals and birds).

The "zoonotic" infectious diseases cycle naturally in animal populations. Transmission to humans occurs when humans encroach on the cycle or when there is environmental disruption, including ecological and meteorological factors. Various rodent-borne diseases, for example, are dependent on environmental conditions and food availability that determine rodent population size and behavior. An explosion in the mouse population following extreme rainfall from the 1991-1992 El Niño event is believed to have contributed to the first recorded outbreak of hantavirus pulmonary syndrome in the United States (Engelthaler et al., 1999; Glass et al., 2000).

Many important infectious diseases, especially in tropical countries, are transmitted by vector organisms that do not regulate their internal temperatures and therefore are sensitive to external temperature and humidity (see Table 9-1). Climate change may alter the distribution of vector species—increasing or decreasing the ranges, depending on whether conditions are favorable or unfavorable for their breeding places (e.g., vegetation, host, or water availability). Temperature also can influence the reproduction and maturation rate of the infective agent within the vector organism, as well as the survival rate of the vector organism, thereby further influencing disease transmission.

Changes in climate that will affect potential transmission of infectious diseases include temperature, humidity, altered rainfall, and sea-level rise. It is an essential but complex task to determine how these factors will affect the risk of vector- and rodent-borne diseases. Factors that are responsible for determining the incidence and geographical distribution of vector-borne diseases are complex and involve many demographic and societal—as well as climatic—factors (Gubler, 1998b). An increase in vector abundance or distribution does not automatically cause an increase in disease incidence, and an increase in incidence does not result in an equal increase in mortality (Chan et al., 1999). Transmission requires that the reservoir host, a competent arthropod vector, and the pathogen be present in an area at the same time and in adequate numbers to maintain transmission. Transmission of human diseases is dependent on many complex and interacting factors, including human population density, housing type and location, availability of screens and air conditioning on habitations, human behavior, availability of reliable piped water, sewage and waste management systems, land use and irrigation systems, availability and efficiency of vector control programs, and general environmental hygiene. If all of these factors are favorable for transmission, several meteorological factors may influence the intensity of transmission (e.g., temperature, relative humidity, and precipitation patterns). All of the foregoing factors influence the transmission dynamics of a disease and play a role in determining whether endemic or epidemic transmission occurs.

The resurgence of infectious diseases in the past few decades, including vector-borne diseases, has resulted primarily from demographic and societal factors—for example, population growth, urbanization, changes in land use and agricultural practices, deforestation, international travel, commerce, human and animal movement, microbial adaptation and change, and breakdown in public health infrastructure (Lederberg et al., 1992; Gubler, 1989, 1998a). To date, there is little evidence that climate change has played a significant role in the recent resurgence of infectious diseases.

The following subsections describe diseases that have been identified as most sensitive to changes in climate. The majority of these assessments rely on expert judgment. Where models have been developed to assess the impact of climate change, these also are discussed.

Table 9-2: Effect of climate factors on vector- and rodent-borne disease transmission.
Climate Factor	Vector	Pathogen	Vertebrate Host and Rodents
Increased temperature	Decreased survival, e.g., Culex. tarsalis (Reeves et al., 1994) Change in susceptibility to some pathogens (Grimstad and Haramis, 1984; Reisen, 1995); seasonal effects (Hardy et al., 1990) Increased population growth (Reisen, 1995) Increased feeding rate to combat dehydration, therefore increased vector-human contact Expanded distribution seasonally and spatially	Increased rate of extrinsic incubation in vector (Kramer et al., 1983; Watts et al., 1987) Extended transmission season (Reisen et al., 1993, 1995) Expanded distribution (Hess et al., 1963)	Warmer winters favor rodent survival
Decreases in precipitation	Increase in container-breeding mosquitoes because of increased water storage Increased abundance for vectors that breed in dried-up river beds (Wijesunder, 1988) Prolonged droughts could reduce or eliminate snail populations	No effect	Decreased food availability can reduce populations Rodents may be more likely to move into housing areas, increasing human contact
Increases in precipitation	Increased rain increases quality and quantity of larval habitat and vector population size Excess rain can eliminate habitat by flooding Increased humidity increases vector survival Persistent flooding may increase potential snail habitats downstream	Little evidence of direct effects Some data on humidity effect on malarial parasite development in Anopheline mosquito host	Increased food availability and population size (Mills et al., 1999)
Increase in precipitation extremes	Heavy rainfall events can synchronize vector host-seeking and virus transmission (Day and Curtis, 1989) Heavy rainfall can wash away breeding sites	No effect	Risk of contamination of flood waters/runoff with pathogens from rodents or their excrement (e.g., Leptospira from rat urine)
Sea-level rise	Coastal flooding affects vector abundance for mosquitoes that breed in brackish water (e.g., An. subpictus and An. sundaicus malaria vectors in Asia)	No effect	No effect

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