The transition of a person from a nomadic lifestyle to a sedentary one that began in the Neolithic led to his gradual acquisition of almost the entire modern set of parasites and pathogens. Among these parasites, fleas are of paramount importance as carriers of plague, one of the most dangerous diseases on the planet. Fleas are constant companions of people. Pets, like humans themselves, are primarily permanent power sources for fleas. The book in a popular form tells about the history of the study of fleas, their structure, life cycle, lifestyle, diet, epidemics (pandemics) of plague, which have covered the world many times and claimed hundreds of thousands of people.