When Old World diseases were first carried to the Americas at the end of the fifteenth century, they spread throughout the southern and northern hemispheres, leaving the indigenous populations in near ruins. No evidence has been discovered that the earliest Spanish colonists and missionaries deliberately attempted to infect the American natives, and some effort was actually made to limit the devastating effects of disease before it killed off what remained of their forced slave labor under their encomienda system. The cattle introduced by the Spanish contaminated various water reserves which Native Americans dug in the fields to accumulate rainwater. In response, the Franciscans and Dominicans created public fountains and aqueducts to guarantee access to drinking water. But when the Franciscans lost their privileges in 1572, many of these fountains were no longer guarded and so deliberate well poisoning may have happened. Although no proof of such poisoning has been found, some historians believe the decrease of the population correlates with the end of religious orders' control of the water.