Study reveals how the evolution of a single gene allowed the plague to keep killing

In the largest mass-mortality event ever recorded, the Black Death swept through Europe in the 14th century, killing between 30 to 50 per cent of the human population. After the initial wave, the plague persisted for more than 500 years in a series of smaller epidemics. History suggests that the bacterium that causes the plague changed as it ran its course, becoming less deadly and potentially allowing the disease to infect more hosts.