Most infectious diseases of the urinary system in small animals are aerobic bacterial infections. Common organisms include Escherichia coli, Staphylococcus, Enterococcus, and Streptococcus. Less common organisms causing infection include Klebsiella, Proteus, and Pseudomonas. Pasteurella is more common in cats than dogs. Mycoplasma is an uncommon cause of urinary tract infection and is usually found as a coinfection with bacteria. Leptospirosis is a worldwide zoonotic disease caused by filamentous bacteria that infect the kidney and many other organs. Fungi, yeast, and parasites uncommonly infect the urinary system.