In 1528, one of two barges put together by survivors of the failed Pánfilo de Narváez expedition to Florida struck aground at Galveston Island. Survivors, including Cabeza de Vaca, were cared for by the Capoque band of Karankawa. From 1527, Cabeza de Vaca subsisted for seven years among the coastal tribes, making a living as a medical practitioner and occasional trader. During his stay, de Vaca reported that a fatal stomach ailment reduced the Karankawa population by roughly one half; the nature and casualties resulting from this illness are unknown. De Vaca reported that extensive trade occurred with inland groups as far as the extent of the entire length of the present-day United States. After the introduction of the horse by Spaniards, these trade networks strengthened.