This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
[{'content': 'Carriers are important in all the following except- \nSelect and explain. \nA. Polio, B. Typhoid, C. Measles, D. Diphtheria', 'role': 'user'}, {'content': 'Answer: C \nExplanation: The only source of infection of measles is a case of measles. Carriers are not known to occur. There is some evidence to suggest that sub clinical measles occurs more often than previously thought.', 'role': 'assistant'}]