"Infant" almost certainly means before they would've been given a measles vaccine, which would mean they are the victim of other people not being vaccinated and the general decline of population immunity.
One of the two people who recently died of measles in Pennsylvania was an infant, the state Health Department said Wednesday.
The department announced Tuesday that two unvaccinated residents had died of measles, the first measles-related deaths in the U.S. this year and the first in Pennsylvania in 35 years.
...
Both were residents of Lancaster County, where the share of kids who have gotten the MMR vaccine fell from 95.5% in 2017 to 87.6% last year.