Better understanding of how environmental factors influence human disease risk increases the likelihood of averting future infectious disease outbreaks or pandemics. In the Middle Atlantic region of the United States, tick-borne disease has become a significant public health burden. Lyme disease (LD) is the most commonly reported vector-borne disease in the USA, with almost 20,000 cases reported last year. The LD agent and many other pathogens are carried and transmitted to humans by Ixodes scapularis, the “black-legged,” or “deer” tick. The most predictive meteorological indicator for human LD risk in this region is late spring/early summer precipitation (McCabe and Bunnell, 2004). Infection prevalence with the LD agent among tick populations has been measured, and tick population density patterns can indicate areas at greater or lesser risk for human exposure. In a geographic information system (GIS) with spatial statistical analysis, strong correlation was observed between adult tick abundance and soils with certain characteristics in this region (e.g., sandy, acidic). However, some soil factors that were significantly associated with tick abundance in a dry year were not so correlated in a wet year. It is hypothesized that an interaction between soil type and precipitation affects tick abundance patterns, probably by affecting tick survival and reproductive success. For instance, a poorly drained soil in a dry year may create microclimatic conditions similarly suitable for tick populations as a well drained soil in a wet year. It has been suggested that such environmental factors exert their impact on human LD cases indirectly by influencing habitat suitability for the ticks' reservoir hosts such as mice and deer. However, because ticks spend the vast majority of their lives in the top layer of soil, rather than on vertebrate hosts, it is reasonable to expect that the effects of such environmental parameters are also direct. For instance, soil pH could affect the accessibility of trace metal nutrients to tick eggs deposited in the A horizon. Similarly, a certain moisture regime is required for proper tick development: tick eggs will be colonized and destroyed by fungi or will desiccate when humidity is too high or too low, respectively. Pending studies will investigate whether hypothesized interactions between precipitation and soil type are correlated with spatiotemporal patterns of tick abundance, and hence Lyme (and other tick-borne) disease risk. McCabe GJ, Bunnell JE. 2004. Precipitation and the Occurrence of Lyme Disease in the Northeastern United States, Vector-Borne and Zoonotic Diseases, 4 (2):143-148.