Wanna bet? Your Wikapedia definition is factually incorrect.
Herd immunity
"A level of resistance in a herd or flock which is sufficient to prevent the entry of a particular disease into, or its spread within, the herd. The resistance
may be innate, a
genetically based resistance, or acquired as a
result of previous exposure to the particular agent
or of vaccination.
http://medical-dictionary.thefreedictionary.com/Herd+Immunity
Thus, herd immunity may be and perhaps mostly is the result of previous infections to some of the population. In the case of Polio, many more people acquired Polio but had no symptoms, and thus were able to pass on immunity to others without having the disease even diagnosed. Fact is, at the time of the initial mass vaccinations, Polio, was already in decline due to herd immunity.