**Core Concept**
The plague, caused by Yersinia pestis, is transmitted to humans through the bites of infected fleas or direct contact with infected animals. There are three primary forms of the plague: bubonic, pneumonic, and septicemic. Fleas are the primary vectors responsible for transmitting the disease.

**Why the Correct Answer is Right**
The correct answer includes all three vectors of plague transmission: X. cheopis (the Oriental rat flea), X. astia (the tropical rat flea), and Pulex irritans (the human flea). X. cheopis and X. astia are the primary vectors responsible for transmitting bubonic plague, while Pulex irritans can transmit the disease to humans through direct bites. These fleas acquire the bacteria by feeding on infected rodents, which allows them to transmit the disease to humans.

**Why Each Wrong Option is Incorrect**
**Option A:** X. cheopis is a primary vector of plague transmission, but it is not the only one. X. cheopis is responsible for transmitting the disease to rodents, which are then bitten by other fleas that transmit the disease to humans.

**Option B:** X. astia is also a primary vector of plague transmission, similar to X. cheopis. However, the correct answer includes all vectors, not just one.

**Option C:** Pulex irritans is a human flea that can transmit the plague, but it is not the only vector responsible for transmitting the disease. The correct answer includes all vectors, not just Pulex irritans.

**Clinical Pearl / High-Yield Fact**
Fleas are responsible for transmitting the plague to humans, and the disease can be prevented by controlling the flea population through measures such as insecticides, traps, and rodent control.

**✓ Correct Answer: D. All of the above**