Simplify the following text for a Grade 9 native English-speaking person.
Original: Bubonic plague was often used synonymously for plague , but it does in fact refer specifically to an infection that enters through the skin and travels through the lymphatics , as is often seen in flea-borne infections .
Simplified: Bubonic plague is the best-known form of the disease plague , which is caused by the bacterium Yersinia pestis . The name bubonic plague is specific for this form of the disease , which enters through the skin , and travels through the lymphatic system .