This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Concept of health and disease, Subject name: Social & Preventive Medicine
[{'content': 'Epidemiological triad includes all except \nSelect and explain. \nA. Host, B. Environment, C. Agent, D. Investigator', 'role': 'user'}, {'content': "Answer: D \nExplanation: The Epidemiologic Triad consists of an external agent, a host and an environment in which host and agent are brought together, causing the disease to occur in the host. Ref: 25th edition, Park's Textbook of Preventive and Social Medicine, Page no. 39", 'role': 'assistant'}]