The story
Rocky Mountain refers to the US Rocky Mountains, where the disease was first identified. Spotted fever refers to the rash and the fever that are the most obvious symptoms.
Etymology from Wiktionary, CC BY-SA 4.0.
Rocky Mountain spotted fever
Today it means: A disease of the Americas characterized by fever and rashes, caused by bacteria of the species Rickettsia rickettsii and transmitted by ticks. Full definition →