You would have to read the article, VCifuentes. Immunization for many communicable diseases does not effectively exist if the immunized person lives in an un-immunized population. Very high numbers -  70% to 95% of the population (the "herd") - must be immunized.

It seems likely to me, a layperson in the matter, that urban and suburban population densities are not possible without immunization and other public health programs like water treatment. Ants seem to be able to live in very high population densities, but humans and other large mammals and birds cannot. Ask any farmer about farm animals, and what a risk it is to bring a non-immunized animal into the herd. If the herd is not nearly completely immunized, and the new animal is carrying a communicable disease, the herd could all be gone in a month.

300 people on an airplane or in a seniors long term care home, a hundred children in a school, a few thousand wandering a mall, tourist site, or religious convention, are all herds.