<prompt> Which of the following is the best tool to measure Communicability of an infection?Prevalence ratePrimary attack rateSecondary attack rateCarrier rate  <response>: Secondary attack rate SAR is defined as the number of exposed persons developing the disease within the range of the incubation period, following exposure to the primary case. SAR No. of exposed persons developing the disease within the range of the incubation period x 100 --- Total no. of susceptible contacts The primary case is excluded from both the numerator and the denominator. It measures communicability infectivity of an infection. SAR of Measles 90; Mumps 86; Chicken pox 90. 