Plague carriers are often associated with rats, which is why the crossword solution for the term 'plague carriers' is RATS. Rats have been known to carry and spread the bubonic plague, a deadly disease that killed millions of people in Europe during the Middle Ages. They can also carry other diseases such as leptospirosis, salmonellosis, and rat-bite fever. Rats are also capable of spreading fleas, which can further spread the plague.