**Core Concept**
Salmonella typhi is a Gram-negative bacterium that causes typhoid fever, a systemic infection primarily affecting the gastrointestinal tract. The infective dose refers to the minimum number of bacteria required to cause infection in a susceptible host.

**Why the Correct Answer is Right**
The infective dose of S. typhi is typically considered to be around 10^3 to 10^6 organisms. This is because S. typhi is able to invade the intestinal mucosa and cause infection even at relatively low doses. The bacteria are able to adhere to the intestinal epithelial cells using their type 1 fimbriae, and then invade the cells using a type III secretion system. The infective dose is often influenced by factors such as the virulence of the strain, the immune status of the host, and the presence of any pre-existing gut flora.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the infective dose of S. typhi is generally considered to be higher than 10 organisms.
* **Option B:** This option is incorrect because there is no established minimum infective dose for S. typhi.
* **Option D:** This option is incorrect because the infective dose of S. typhi is not typically measured in millions of organisms.

**Clinical Pearl / High-Yield Fact**
It's worth noting that the infective dose of S. typhi can vary depending on the mode of transmission. For example, the infective dose is typically lower for ingestion of contaminated food or water, compared to inhalation of aerosolized bacteria.

**Correct Answer:** C. 10^3 to 10^6 organisms.