The World Health Organization said on Monday that an outbreak of Ebola in West Africa has been largely contained in Senegal and Nigeria, but the disease is still spreading elsewhere and has now killed over 2,811 people in the region. Senegal and Nigeria, the most recent of five nations to record cases of Ebola, implemented strict measures to isolate the ill and track down further possible cases, steps that Guinea, Liberia and Sierra Leone have failed to impose, allowing the disease to take hold in cities and rural communities. The current outbreak was first identified in the forests of southeastern Guinea in March and then spread into neighboring Liberia and Sierra Leone, where it overwhelmed weak national health systems.