The potentially deadly strain, believed to have been brought to Haiti by UN workers helping recovery efforts after the island nation’s deadly earthquake three years ago, is spreading. And officials aren’t sure when, or where, it might end. The disease is difficult to halt once it sneaks into areas with bad sanitation and water supplies. Mexico has tallied 171 cases since Sept. 9, while Cuba reports almost 700. Other nations where travelers might have visited or returned to after a stop in Haiti could also be at risk.