**Core Concept**
Salmonella typhi, the causative agent of typhoid fever, is a gram-negative bacterium that primarily infects the gastrointestinal tract. The infective dose of S. typhi is a critical factor in understanding the pathogenesis of typhoid fever.

**Why the Correct Answer is Right**
The infective dose of S. typhi is estimated to be around 10^3 to 10^6 organisms. This is relatively low compared to other pathogens, which is why typhoid fever can be transmitted through contaminated food and water. The bacteria invade the intestinal mucosa, causing inflammation and eventually leading to systemic infection. The exact mechanism of invasion involves the expression of type III secretion system (T3SS) proteins, which help the bacteria to evade the host's immune response.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the infective dose of S. typhi is not in the range of 10^9 to 10^12 organisms, which is more typical of other pathogens.
**Option B:** This option is incorrect because the infective dose of S. typhi is not directly related to the number of organisms required to cause bacteremia.
**Option C:** This option is incorrect because the infective dose of S. typhi is not determined by the virulence of the strain.

**Clinical Pearl / High-Yield Fact**
It's worth noting that the infective dose of S. typhi can vary depending on factors such as the host's immune status and the presence of underlying conditions.

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