Plague is an ancient disease which has caused epidemics of illness and the deaths of millions of people throughout history. During the “Black Death” of the 14th century, an estimated one-third of the population of Europe died from plague. Today, plague is rare among humans but is found each year among squirrels, chipmunks, and other rodents in California and the southwestern U.S. People can be infected with plague bacteria if they are bitten by a flea from an infected rodent or have close contact with an infected animal. Most people with plague develop fever and swollen lymph nodes. Plague is treatable with antibiotics, but can progress to severe and sometimes fatal illness if diagnosis and treatment are delayed. People visiting, hiking, camping, or living in areas where plague occurs should avoid contact with wild rodents and their fleas. Domestic cats are also susceptible to plague and can pass the infection to their owners.