**Core Concept**
Pediculus humanus, the human louse, is a ectoparasite that transmits various diseases to humans. The correct answer involves understanding the vector-borne transmission of a specific disease.

**Why the Correct Answer is Right**
Chaga's disease, also known as American trypanosomiasis, is caused by the protozoan parasite Trypanosoma cruzi. Pediculus humanus is the primary vector for transmitting this disease. When an infected louse defecates, it can deposit T. cruzi in its feces near the bite site, leading to infection in humans. This mode of transmission highlights the importance of vector control in preventing Chaga's disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** Sleeping sickness, caused by Trypanosoma brucei, is transmitted by the tsetse fly (Glossina spp.), not Pediculus humanus.
* **Option B:** Plague, caused by Yersinia pestis, is primarily transmitted through the bites of infected fleas (Xenopsylla cheopis), rat bites, or respiratory droplets, not Pediculus humanus.
* **Option D:** Relapsing fever, caused by Borrelia recurrentis, is transmitted by the bite of infected lice, but it is not the primary vector; the primary vector for relapsing fever is the soft tick (Ornithodoros spp.).

**Clinical Pearl / High-Yield Fact**
It's essential to recognize that Pediculus humanus is the primary vector for Chaga's disease, highlighting the importance of personal hygiene, vector control, and public health measures in preventing this disease.

**✓ Correct Answer: C. Chaga's disease**