The word "undulate" comes from the Latin "undula" which is the diminutive of "unda" meaning wave. So "undula" is a little wave. The adjective is "undulant."
Undulant fever, also called brucellosis, is an infectious disease characterized by a rising and falling (undulant) fever. It is caused by a bacterium called Brucella that can be transmitted in unpasteurized milk or cheese of cattle, sheep, and goats or by contact with diseased animals.