**Core Concept**
The infective dose of Salmonella Typhi represents the number of bacteria required to initiate infection in a susceptible individual. This concept is crucial in understanding the pathogenesis of typhoid fever, a disease caused by S. Typhi.

**Why the Correct Answer is Right**
The infective dose of S. Typhi is estimated to be between 10^3 to 10^6 bacteria. This small number is sufficient to cause infection due to the bacteria's ability to invade and replicate within the intestinal epithelial cells. Once inside, S. Typhi can evade the host's immune response and establish a systemic infection. The bacterium's virulence factors, such as the Vi capsular antigen and the Salmonella pathogenicity island (SPI), play a crucial role in its ability to infect and colonize the host.

**Why Each Wrong Option is Incorrect**
**Option A:** 10 bacilli is an extremely low number, and it is unlikely that a single bacterium would be sufficient to cause infection in a healthy individual.
**Option B:** 1000 bacilli is still a relatively low number, but it is closer to the estimated infective dose. However, it is not the correct range.
**Option D:** 10^10 - 10^12 bacilli is an excessively high number, far exceeding the estimated infective dose.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the infective dose of S. Typhi can vary depending on factors such as the individual's immune status, the presence of underlying health conditions, and the virulence of the bacterial strain. However, the estimated range of 10^3 to 10^6 bacteria remains a useful benchmark for understanding the pathogenesis of typhoid fever.

✓ Correct Answer: C. 10^3-10^6 bacilli