## Core Concept
The infective dose of *Salmonella Typhi* is the minimum number of bacteria required to cause infection in a human host. This dose is critical in understanding the epidemiology and transmission dynamics of typhoid fever. The infective dose can vary depending on several factors, including the form of the bacteria (e.g., ingestion of contaminated food or water) and the host's immune status.

## Why the Correct Answer is Right
The correct answer, , indicates that a very small number of *S. Typhi* bacteria, specifically fewer than 1000, can cause infection. This low infective dose is one reason why typhoid fever can spread rapidly in areas with poor sanitation and hygiene. The bacteria can survive passage through the stomach and establish infection in the intestines, from where they can invade the bloodstream and cause systemic disease.

## Why Each Wrong Option is Incorrect
* **Option A:** - This option suggests a much higher infective dose than what is established for *S. Typhi*. While the exact number can vary, is considered too high for the infective dose of typhoid.
* **Option B:** - Similarly, this option also represents a higher infective dose than the established range for *S. Typhi*.
* **Option D:** - This option suggests an even higher number, which is not consistent with the known low infective dose of *S. Typhi*.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the infective dose of *S. Typhi* is very low, which underscores the importance of good hygiene practices and safe food and water handling in preventing the spread of typhoid fever. This is particularly crucial in endemic areas or during outbreaks.

## Correct Answer Line
**Correct Answer: C. .**