## **Core Concept**
The **X** cheopis index, also known as the "cheopis index," refers to the average number of *Xenopsylla cheopis* (the Oriental rat flea) per rodent. This index is a critical indicator in the control of plague epidemics, as *Xenopsylla cheopis* is a primary vector of the bubonic plague bacterium, *Yersinia pestis*. Effective vector control measures, such as the application of insecticides, are essential to reduce the transmission of the disease.

## **Why the Correct Answer is Right**
The correct answer indicates that within 48 hours of applying insecticides, the cheopis index should fall to 0. This rapid reduction in the flea index is a benchmark for the effectiveness of the vector control measures being implemented during a plague epidemic. A significant decrease in the flea population within a short period signifies successful interruption of the disease transmission cycle, thereby helping to control the spread of the plague.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests a cheopis index of 1 or more, indicating that the flea population has not been adequately reduced. This level of infestation would still pose a significant risk for the transmission of the plague.
- **Option B:** A cheopis index of 0.5 implies that there is still a considerable flea population present on rodents, which could sustain the transmission of the disease.
- **Option D:** A cheopis index of 2 or more would indicate a high level of flea infestation, suggesting that the insecticide application has not been effective in controlling the vector population.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that rapid reduction of the vector population is crucial in controlling plague epidemics. The cheopis index is a valuable tool for monitoring the effectiveness of vector control measures. Aiming for a quick drop in the index, such as to 0 within 48 hours of insecticide application, helps ensure that the risk of disease transmission is minimized.

## **Correct Answer:** . 0.