**Question:** According to International Health Regulations, there is no risk of spread of yellow Fever if the Aedes aegypti index remains below

A. 0.5
B. 1
C. 1.5
D. 2

**Core Concept:** Yellow fever is a viral hemorrhagic fever caused by the Yellow Fever virus, primarily transmitted to humans through the bite of infected mosquitoes, particularly the Aedes aegypti mosquito. The International Health Regulations (IHR) are international guidelines aimed at preventing and alerting to outbreaks of diseases, including yellow fever.

**Why the Correct Answer is Right:** According to the IHR, if the Aedes aegypti index remains below 2, there is no significant risk of yellow fever transmission. An Aedes aegypti index refers to the number of Aedes aegypti mosquitoes per 100 houses, with a value of 2 or higher indicating increased risk of transmission.

**Why Each Wrong Option is Incorrect:**

A. 0.5 - This option is too low compared to the threshold of 2 set by the IHR. A low index value indicates a lower risk of transmission, not no risk.

B. 1 - Similar to option A, 1 is too close to 2 and does not guarantee no risk of transmission.

C. 1.5 - This option is also too close to 2 and does not guarantee no risk of transmission.

**Why the Correct Answer is Right:** Option D (2) is the correct answer because it represents the threshold below which the risk of yellow fever transmission is considered minimal due to a low Aedes aegypti index.

**Clinical Pearl:** Understanding the International Health Regulations and the Aedes aegypti index is crucial for healthcare professionals working in areas with potential yellow fever transmission. Aedes aegypti is a disease vector for other diseases as well, such as dengue fever and Zika virus. Staying up-to-date with the guidelines and transmission risks helps in preventing and controlling outbreaks of vector-borne diseases.