<prompt> All of the following about herd immunity are true, except:Spread of epidemic is influenced by itCan be acquired by immunisationIt is mostly due to sub-clinical infectionHerd structure is constant  <response>: 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: Parks Textbook of Preventive and Social Medicine, 19th Edition; Pages 96 - 97 