**Core Concept**
Chronic carriers of typhoid shed Salmonella Typhi bacilli in their feces for extended periods, posing a significant risk of transmission to others. This phenomenon is attributed to the organism's ability to persist in the gallbladder, where it can evade the host's immune system.

**Why the Correct Answer is Right**
Chronic carriers of typhoid typically shed bacilli for 1-3 years, although some may continue to excrete the organism for up to 20-30 years. The prolonged shedding is thought to be due to the formation of a mucosal reservoir in the gallbladder, where the bacteria can remain viable despite the presence of a functioning immune system. This reservoir allows the organism to periodically infect the intestinal tract, leading to continued shedding of bacilli in the feces.

**Why Each Wrong Option is Incorrect**
**Option A:** 6 months - This duration is typically associated with the acute phase of typhoid fever, not chronic carriage.
**Option B:** 10-15 years - While some carriers may continue to shed bacilli for this extended period, it is not a universally accepted duration for chronic carriage.
**Option C:** Lifetime - This option is overly broad and not supported by scientific evidence.

**Clinical Pearl / High-Yield Fact**
To diagnose chronic carriers of typhoid, stool cultures are often performed, as these individuals can intermittently shed bacilli in their feces. A single positive culture in a person with a history of typhoid fever is sufficient to establish the diagnosis of a chronic carrier.

**Correct Answer:** C.