Don't infants have natural immunity?
Babies get some temporary immunity (protection) from mom during the last few weeks of pregnancy—but only for the diseases mom is immune to. These antibodies do not last long, leaving the infant vulnerable to disease.
Haven't we gotten rid of most of these diseases in this country?
Thanks to vaccines, most diseases prevented by vaccines are no longer common in this country. Even the few cases we have in the U.S. could very quickly become tens or hundreds of thousands of cases if we stopped vaccinating.
It's not uncommon to have measles outbreaks, whooping cough outbreaks, chickenpox outbreaks, and other diseases when vaccination rates drop. Kids that are not fully vaccinated can become seriously sick and spread it through a community.