Flea bites. In areas of poor hygiene/sanitation; where rats congregate, as it is the flea from the rat that transmits the yersinia pestis to humans. Low income areas; those damaged by natural or man-made disasters.
Bites; air droplets. The plague is usually bubonic (one catches it from being bit by infected fleas or rodents, and the bacteria infects one's lymph nodes) or pneumonic (one catches it by breathing infected moisture droplets exhaled, sneezed, or coughed out by an infected person or animal, and the bacteria infects one's lungs). The chances of survival without treatment is about 50% for bubonic, and zero for pneumonic.