A flea is a small creature with no wings but is capable of jumping incredible heights.
A lot of cat carry the bacteria responsible for cat scratch fever, and most do not show any signs. Although this creature lacks the ability to survive on its own, it is only capable of surviving as an external parasite on the body of mammals and birds. hmm.
Plague would not be as common of an issue and is a more rare disease in humans. Cat scratch fever, plague and typhus are diseases that can be transmitted by fleas to cats and humans. Fleas on people? The adult fleas you see are only 5% of the flea population, so controlling the immature forms is a must.