Answer: a) Herd immunityHerd immunity* Vaccination provides excellent protection to a population, even if not every individual in a population is vaccinated, because of a phenomenon known as "herd immunity".* When a great percentage of population is already vaccinated, the infectious agent will face great difficulty to find an unprotected individual and spread the disease.* The population becomes resistant as a whole.* But herd immunity has its own limitations. If a significant number of unvaccinated individuals enter the population and contract the disease, then the infection will spread quickly to the remainder of unvaccinated people. Tolerance is simply defined as unresponsiveness of immune system to an antigen. Genetic drift is the minor genetic change that occurs due to mutations, or changes in the genetic material of a population sample. Immune evasion is a mechanism by which microbes and tumors evade host's immune response. Genetic shift is the major genetic change. It is a process by which a completely new strain is formed due to combination of two or more different strains (of viruses).