## Core Concept
Typhoid fever is a bacterial infection caused by *Salmonella Typhi*, primarily affecting the gastrointestinal system. It is mainly spread through contaminated food and water. Understanding its transmission, diagnosis, and prevention is crucial for managing the disease.

## Why the Correct Answer is Right
The correct answer involves verifying each statement about typhoid:
- *c) Ty21a is an oral vaccine*: This is true. The Ty21a vaccine is a live, attenuated oral vaccine used for the prevention of typhoid fever. It's a well-established method for immunization against *Salmonella Typhi*.

## Why Each Wrong Option is Incorrect
- **Option A:** *a) It is caused by food poisoning* - This statement can be misleading. Typhoid is caused by *Salmonella Typhi*, not typically classified under "food poisoning," which usually refers to infections caused by other bacteria like *Staphylococcus*, *Bacillus cereus*, or *Clostridium perfringens*.
- **Option B:** *b) Water can transmit the disease* - While true, this option does not directly contribute to identifying the correct answer without evaluating the others.
- **Option D:** *d) Chronic carrier is called when transmitted upto 6 month* - This statement seems inaccurate. A chronic carrier of typhoid is typically defined as someone who continues to harbor *Salmonella Typhi* in their gallbladders for more than a year, often for many years, not just up to 6 months.
- **Option E:** *e) Widal test +ve in 1st week* - The Widal test, used for diagnosing typhoid, typically becomes positive in the second week of illness, not the first. Early in the disease, the test may be negative.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the *Ty21a vaccine* is an oral, live-attenuated vaccine that provides immunity by inducing a local immune response in the gut, which is the primary site of *Salmonella Typhi* infection.

## Correct Answer: C.