Nasty thing that. Bubonic plague became famous for killing nearly half the western world in the 14th century in just a few years, but for all its voracious destructiveness, the pneumonic variant left it in the dust in specific situations. I've read that in cities and towns where plague took on its pneumonic form instead of its bubonic variant, 80%+ of the local population would die in just days. In some cities struck by this, populations didn't recover until the 18th century.