This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Infectious Disease Epidemiology, Subject name: Social & Preventive Medicine
Measure of lnfectivity of a disease: 
Select and explain. 
A. Incidence rate, B. Prevalance rate, C. Secondary attack rate, D. Case Fatality rate
C: Secondary attack rate
Answer: C: Secondary attack rate 
Explanation: Ref: Park 'Textbook of Preventive and Social Medicine', 22nd edition, Pg: 58, 55, 96Explanation:Incidence rateNumber of new cases occurring in a defined population during a specified period in timePrevalence rateIt includes old and new cases, it depends on Incidence and duration of disease Prevalence = incidence X durationSecondary attack rateIndicates communicability or infec- tivrtyNo. of contacts d eve lo Dina disease in one maximum IP X 100 Total no. susceptible contactsCase fatality rateSeverity of the disease, killing power of the diseaseIndicators of severity of disease1. Case fatality rate2. Disability rate