**Core Concept**
Scrub typhus is a vector-borne disease caused by the bacterium Orientia tsutsugamushi, transmitted through the bite of infected chiggers (larval mites).

**Why the Correct Answer is Right**
The correct answer is transmitted by chiggers, which are the larval stage of the mite family Trombiculidae. Chiggers inject the bacteria into the skin during feeding, leading to infection. The bacteria then multiply and cause the characteristic symptoms of scrub typhus, including fever, headache, and rash.

**Why Each Wrong Option is Incorrect**
* **Option A:** is incorrect as scrub typhus is not transmitted by mosquitoes, which are the vectors for diseases like dengue and Zika.
* **Option B:** is incorrect as ticks are the vectors for diseases like Lyme disease and Rocky Mountain spotted fever, not scrub typhus.
* **Option C:** is incorrect as fleas are the vectors for diseases like plague, not scrub typhus.

**Clinical Pearl / High-Yield Fact**
Scrub typhus is often misdiagnosed as leptospirosis or dengue fever, so a high index of suspicion is necessary for diagnosis.

**Correct Answer: D. Chiggers.**