Six days after receiving several flea bites in a rat-infested shed, a homeless 24-year-old-man develops fever, chills, and a rash that spreads from his abdomen to cover his extremities. He is seen at the hospital emergency depament, where blood is drawn for analysis. Eight days later, the public health depament repos the presence of antibody to one of the rickettsial group antigens. Which of the following is the most likely diagnosis?