An outbreak of what is thought to be pneumonic plague has spread to a second town in the Democratic Republic of Congo. Pneumonic plague occurs when the Yersinia pestis bacteria infects the lungs. It has a very high mortality rate and can be transmitted from person to person by way of respiratory droplets. The outbreak began days after a diamond mine re-opened north of the country's biggest city. Sixty people have died so far and all have been diamond miners. Another 350 miners have been infected. Experts from the World Health Organization has visited the area to confirm that people are infected with the plague.