The parasites don't have to follow, they just expand the city limits and re-absorb the escaped hosts via annexation.
It depends on the state. St. Louis City, for instance, is deep navy blue (81% for Kerry this election), but only has about 350,000 people.
Neighboring St. Louis and St. Charles County between them have about 1.5 million people. The city is always whining about forcing the neighboring counties into one big megalopolis, but guess what - our state constitution mandates a vote. Since the population differences are so skewed, something like that would never pass, at least not here. Of course, if a city can annex without a vote, then that state has far worse problems.