About herd immunity, all are true except -

Ans. is 'd' i.e., Herd immunity is constant Elements which contribute to herd immunity :? a) Occurence of clinical and subclinical infection in the herd -4 Clinical or subclinical infection may cause development immunity to that paicular disease. b) Immunization of herd --> By vaccination of herd through on-going immunization programme. c) Herd structure --> Herd structure is never constant. It is subjected to constant variation because of new bihs (addition of unimmunized person to herd), new deaths (removal of immunized person from the herd) and population mobility (addition or removal of immunized or unimmunized person to herd depending on the population which is mobilizing). The herd structure includes not only the hosts (population) belonging to the herd species but also the presence and distribution of alternative animal host and insect as well as those environmental and social factors that our or inhibit the spread of infection from host to host. o So, herd immunity is not constant because of continous changes in these elements.