**Question:** To prevent Yellow Fever, the Aedes aegypti index should be less than

A. 1,000
B. 100
C. 10
D. 1

**Core Concept:** The Aedes aegypti index represents the number of households with breeding sites for the Aedes aegypti mosquito, which transmits Yellow Fever virus. A lower index indicates a lower risk of Yellow Fever transmission.

**Why the Correct Answer is Right:** Dengue, Chikungunya, and Yellow Fever are transmitted by the Aedes mosquito species. Among them, Yellow Fever is the most severe disease and requires immediate intervention to control its spread. Reducing the Aedes aegypti index helps to prevent the proliferation of these mosquitoes and thus reduces the risk of Yellow Fever transmission. A lower index indicates fewer households with breeding sites, making the environment less hospitable for the mosquitoes.

**Why Each Wrong Option is Incorrect:**

A. 1,000 (Option A): This option is too high, as a lower index is desirable for disease prevention. A lower index means fewer households with breeding sites, making the environment less conducive for Aedes aegypti proliferation.

B. 100 (Option B): Although higher than Option D, this option is still too high. A lower index is essential for Yellow Fever prevention, and a value of 100 indicates a significant number of households with breeding sites.

C. 10 (Option C): Similar to Option B, 10 is too high. A lower index is crucial for preventing Yellow Fever, and a value of 10 suggests fewer households without breeding sites, still not ideal for prevention.

D. 1 (Option D): This option represents a low Aedes aegypti index, which is desirable for Yellow Fever prevention. Reducing the index to 1 indicates almost no households with breeding sites, making the environment unsuitable for Aedes aegypti proliferation and thus reducing the risk of Yellow Fever transmission.

**Clinical Pearl:** The Aedes aegypti index is essential to consider while planning public health measures for vector-borne disease control. A lower index (1) is crucial for preventing Yellow Fever, Dengue, Chikungunya, and other diseases transmitted by Aedes aegypti mosquitoes. By reducing the index, we can effectively control the spread of these diseases and protect public health.