**Core Concept**
The infective dose of Salmonella Typhi is a critical factor in understanding the epidemiology of typhoid fever. The infective dose refers to the number of bacteria required to cause infection in humans. In the case of S. Typhi, it is known to be a relatively low dose due to its ability to invade and replicate within the intestinal mucosa.

**Why the Correct Answer is Right**
The correct answer is C. 102-105 bacilli. This is because S. Typhi has a relatively low infective dose, typically ranging from 10^2 to 10^5 bacteria. This is due to its ability to invade the intestinal mucosa and replicate within the Peyer's patches, which are part of the immune system in the small intestine. The bacteria can then spread through the bloodstream and cause systemic infection.

The infective dose is influenced by several factors, including the virulence of the strain, the immune status of the host, and the presence of any underlying medical conditions. In general, a higher dose of bacteria is required to cause infection in a healthy individual, whereas a lower dose may be sufficient to cause infection in someone with a compromised immune system.

**Why Each Wrong Option is Incorrect**
**Option A:** One bacillus is incorrect because it is an excessively low dose, and even a single S. Typhi bacterium would not typically cause infection in a healthy individual.

**Option B:** 10^8-10^10 bacilli is incorrect because it is an excessively high dose, far above the typical infective dose required to cause typhoid fever.

**Option D:** 1-10 bacilli is incorrect because it is an excessively low dose, and even a few S. Typhi bacteria would not typically cause infection in a healthy individual.

**Clinical Pearl / High-Yield Fact**
It is worth noting that the infective dose of S. Typhi can vary depending on the mode of transmission, with food and water being more likely to contain higher doses of bacteria than person-to-person transmission. This is an important consideration in public health efforts to prevent the spread of typhoid fever.

**✓ Correct Answer: C. 102-105 bacilli**