 For polio to occur in a population, there needs to be an infecting organism (poliovirus), a susceptible human population, and a cycle of transmission.Poliovirus is transmitted only through person-to-person contact and the transmission cycle of polio is from one infected person to another person susceptible to the disease, and so on.If the vast majority of the population is immune to a particular agent, the ability of that pathogen to infect another host is reduced; the cycle of transmission is interrupted, and the pathogen cannot reproduce and dies out.