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': "All of the following about 'herd immunity' are true, except: \nSelect and explain. \nA. Spread of epidemic is influenced by it, B. Can be acquired by immunisation, C. It is mostly due to sub-clinical infection, D. Herd structure is constant", 'role': 'user'}, {'content': "Answer: D \nExplanation: A herd structure includes the population belong to the herd species, the presence and distribution of altered animal hosts and possible insect vector as well as those environmental and social factors that our or inhibit the spread of infection from host to host. This herd structure can never be constant as it is subjected to new variations. These new variations occur due to new bihs, new deaths and population mobility. Ref: Park's Textbook of Preventive and Social Medicine, 19th Edition; Pages 96 - 97", 'role': 'assistant'}]