This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
[{'content': 'Subclinical cases are seen in all except- \nSelect and explain. \nA. Measles, B. Rubella, C. Polio, D. Influenza', 'role': 'user'}, {'content': "Answer: A \nExplanation: The only source of infection is a case of measles Carriers are not known to occur. There is some evidence to suggest that subclinical measles occurs more frequently than previously thought. Ref: 25th edition, Park's Textbook of Preventive and Social Medicine, Page no. 159", 'role': 'assistant'}]