**Core Concept**
Relapsing fever is a vector-borne disease characterized by recurring episodes of fever, headache, and muscle pain. It is primarily caused by bacteria transmitted through the bites of infected soft-bodied ticks or lice.

**Why the Correct Answer is Right**
Relapsing fever is caused by the bacterium *Borrelia recurrentis*, which is transmitted to humans through the bites of *Pediculus humanus* (human body louse) or soft-bodied ticks. The bacteria multiply in the louse's gut and are then transmitted to humans through feces contaminated with the bacteria. Upon entering the human body, the bacteria cause an infection that leads to symptoms such as fever, headache, and muscle pain. The disease recurs due to the bacteria's ability to evade the host's immune system.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because *Borrelia burgdorferi*, the causative agent of Lyme disease, is not associated with relapsing fever.
**Option B:** This option is incorrect because *Bartonella henselae*, the causative agent of cat-scratch disease, is not related to relapsing fever.
**Option C:** This option is incorrect because *Rickettsia rickettsii*, the causative agent of Rocky Mountain spotted fever, is not associated with relapsing fever.

**Clinical Pearl / High-Yield Fact**
Relapsing fever is a classic example of a vector-borne disease, highlighting the importance of vector control in preventing the spread of infectious diseases.

**Correct Answer: D. Borrelia recurrentis**