And infant mortality rates are measured by percentage of live births, not percentage of people:
http://en.wikipedia.org/wiki/List_of...mortality_rate
And the question typically posed isn't "Why is the US worst?" it's "Why is the US 48th and apparently all other first-world countries better?"
Infant mortality as a benchmark of quality of health care for a nation made sense in the 1950s, before fertility treatment.
Today the benchmark is only used to bash the US system.