The concern is upending family routines, leaving some parents of infants steering clear of public transit, playgrounds, malls and more. For Ms. Traini, 41, who teaches biology at Henry Ford College in Dearborn, going out with Gretha at all felt like a risky move. They live in Oakland County, Mich., just outside Detroit, which in recent weeks has been the site of a major measles outbreak, mostly in Orthodox Jewish areas. At least 43 people have been reported ill in Michigan.

The number of measles cases in the United States has surpassed 700, the highest figure recorded in any year since the disease was declared eliminated in this country in 2000, federal health officials said.

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Many people, vaccinated or not, seem unaware of the particular vulnerability of infants. But current parents of babies have suddenly grown acutely aware, sharing information on playgrounds, in pediatricians’ offices and on Facebook. For many, the days of watching new cases of measles pop up around the country — a reflection of the rise of the anti-vaccination movement and of measles outbreaks in other countries where Americans travel — have been agonizing.

“It’s just maddening, because I shouldn’t have to worry about measles,” said Katherine Jones, 37, a grant manager at an environmental nonprofit group in San Francisco and the mother of an eight-week-old baby.

For people who don’t vaccinate their children, she said, “It’s a choice.”

But she added: “It’s not a choice for me, because my baby cannot be vaccinated. The folks who are choosing not to vaccinate their children or be vaccinated themselves are putting my child in danger.”