Ah another great question and this is a subject that fascinated me right back when I learned about it in my history lessons at school. I was gripped by the idea of all these people dying of this horrible disease and the pictures of strange looking doctors dressed in masks with beaks (a reason why even today we often call doctors “quacks”). In fact the plague has never really gone away, thousands of people are diagnosed with the Plague every year. The difference these days is that we able to contain the disease better and have effective treatments that prevent people from spreading the disease and suffering a horrible death. Also we know how the bug (Y.Pestis is its name) that causes plague is transmitted, it is a bacteria that lives in the digestive tract of fleas and can be passed on to other species when the flea bites an animal or human. Rodents are the most commonly infected animals and it is believed that flee-infested rats coming in on ships from overseas were responsible for bringing in the strain of T Pestis that caused the black death.
If you mean bubonic plague which we know of as “The Plague” or “The Black Death” then it’s never gone away. When it killed lots of people in the past it was because they didn’t know what caused it, what helped it spread or how to cure it. Now we understand exactly what it is and can treat it with antibiotics. That means although it’s never gone away, it will never be as widespread or kill as many people.