Assertion (A) : Mary Mallon continued to spread typhoid for many years. Reason (R) : Salmonella typhi generally enters the small intestine through food and water contaminated with it.

The correct option is: Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).

Let us analyze the statements step-by-step to understand why this option is correct:

1. Analyzing Assertion (A):
Assertion (A) states: "Mary Mallon continued to spread typhoid for many years."
Mary Mallon, famously known as "Typhoid Mary," was a cook in the United States in the early 20th century. She was a healthy, asymptomatic carrier of the bacterium Salmonella typhi, which causes typhoid fever. Despite not showing symptoms herself, she shed the pathogen and spread the disease to numerous people through the food she prepared over several years. Therefore, Assertion (A) is true.

2. Analyzing Reason (R):
Reason (R) states: "Salmonella typhi generally enters the small intestine through food and water contaminated with it."
Typhoid fever is a waterborne and foodborne illness. The causative bacterium, Salmonella typhi, enters the human body via the fecal-oral route. This means it is ingested through food or drinking water that has been contaminated with the feces or urine of an infected person or a carrier. Once ingested, the bacteria pass into the digestive tract and colonize the small intestine, from where they can migrate to other organs through the bloodstream. Therefore, Reason (R) is also true.

3. Link between Assertion (A) and Reason (R):
The reason why Mary Mallon (a cook) was able to spread the disease to others is precisely because Salmonella typhi is transmitted via food and water contaminated by the carrier. Since she prepared food for others without practicing proper hand hygiene, the pathogen from her body contaminated the food, which then entered the small intestines of the people who consumed it, causing them to contract typhoid.
Thus, Reason (R) provides the direct transmission mechanism that explains how Mary Mallon spread typhoid for many years, making it the correct explanation of Assertion (A).