After the Classic Period collapse the population of the area continued to drop dramatically, especially after the introduction of smallpox along with European explorers. The smallpox plague arrived around 1519 or 1520, preceding by several years the first Europeans to visit the region. Hernán Cortés led the first expedition to pass through the Petén Basin, in 1524 to 1525, and reported that the region mostly had small hamlets separated by thick forest, with Tayasal being the only sizable inhabited city they observed.