However, formal infectious disease surveillance systems have important limitations, including lags between case occurrence and reporting. Sentinel sites may report cases only periodically or fail to report altogether for a variety of reasons. Delays in reporting may occur when governmental organizations charged with surveillance aren't able to adequately collect and analyze data or publish reports in a timely manner. These problems may be particularly daunting in developing countries with limited resources to devote to strengthening surveillance systems: robust formal public health surveillance is expensive, requiring major investments in trained personnel, communications, buildings and equipment. Indeed, the economic conditions that prevent development of robust surveillance systems may also be those that potentiate dengue transmission: for example, a seroprevalence study performed in a city straddling the Texas-Mexico border found marked differences in dengue seroprevalence on the Mexican side of the border in association with economic disadvantage [13] .