Thanks to all those shots in the arm, in the year 2000, measles in the United States was declared eliminated. But now, it's coming back, with measles cases reported from California to Vermont . One big reason: across the country in 2023, more families exempted their children from routine immunizations than ever before.

"There's never been a better time in human history to tackle an infectious disease than today," said Dr. Howard Markel, a medical historian, retired from the University of Michigan. "There's so many things we can do, from vaccines to antivirals to antibiotics.

And yet, I am dumbfounded by the volume of anti-vax voices." Markel says vaccine hesitancy is as old as the United States. In the 1700s, when smallpox was ravaging the colonies, some people were given an early form of immunization called variolation.

"You went to a doctor who had this infectious material – dried pus and detritus of smallpox scars and so on," Markel said. "They would cut you open, make a slice of your arm, and inoculate – 'put it in' – your arm. And half of the people got really sick, and some of them died.

So, it cost a lot and it was dangerous." But the people who recovered were immune. Benjamin Franklin decided it was too dangerous for his sickly four-year-old son, Franky.

"One of Franklin's great regrets was that he did not get his son inoculated, instilled with smallpox virus, to prevent what ultimately killed him," Markel said. In the 1800s, as a much safer smallpox vaccine was.