This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Communicable diseases, Subject name: Social & Preventive Medicine
[{'content': 'Infectivity of an organism is measured by \nSelect and explain. \nA. Incidence rate, B. Prevalence rate, C. Secondary attack rate, D. Case fatality rate', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Infectivity of an organism is measured by incidence rate Incidence rate: It is the number of new cases occuring in a defined population during a specified period of time Prevalence rate: It is the number of new and old cases existing in a population during a specified period of time Case fatality rate: It is the ratio of total number of deaths due to a paicular disease to total number of cases due to same disease Secondary attack rate: It is defined as the number of exposed person developing the disease within the range of the incubation period following exposure to a primary case Ref : Park&;s Textbook of preventive and social medicine; 24th edition; Pg no 60', 'role': 'assistant'}]