Rickettsia rickettsii is a type of bacteria that causes Rocky Mountain spotted fever, a serious illness transmitted primarily by ticks. This bacterium is an intracellular parasite, meaning it lives inside the cells of its host. It is commonly found in the Americas, particularly in areas with high tick populations. Infection with Rickettsia rickettsii can lead to symptoms such as fever, headache, and a characteristic rash. If left untreated, it can result in severe complications, including organ failure. Early diagnosis and treatment with antibiotics are crucial for recovery and reducing the risk of serious health issues.