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).

Step-by-step Explanation:

1. Analyzing Assertion (A): "Mary Mallon continued to spread typhoid for many years."
Mary Mallon, famously known as "Typhoid Mary," was a cook in the United States. She was an asymptomatic carrier of the bacterium Salmonella typhi. Because she did not show symptoms herself, she continued to work as a cook and prepared food for many families, thereby unknowingly transmitting the typhoid pathogen to dozens of people over several years. Thus, Assertion (A) is true.

2. Analyzing Reason (R): "Salmonella typhi generally enters the small intestine through food and water contaminated with it."
Typhoid fever is a waterborne and foodborne bacterial infection. The causative pathogen, Salmonella typhi, enters the human body via the fecal-oral route. When someone consumes food or water contaminated with the feces of an infected person or carrier, the bacteria pass through the digestive system and colonize the small intestine, from where they migrate to other organs through the bloodstream. Thus, Reason (R) is true.

3. Linking Assertion (A) and Reason (R):
Why did Mary Mallon continue to spread typhoid for many years? She did so because of her profession as a cook. The bacteria (Salmonella typhi) she carried were shed into the food and water she handled. When others consumed this contaminated food and water, the pathogens entered their small intestines, causing them to contract typhoid fever. Therefore, the mode of entry described in Reason (R) explains the mechanism by which Mary Mallon was able to spread the disease. Consequently, Reason (R) is the correct explanation for Assertion (A).