"Almost all US states allow parents to opt their children out of vaccinations.
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Measles is back in the United States, and to understand why, you have to understand where.

When anti-vaccination misinformation leads to a drop in the number of children immunized, a community can lose its “herd immunity.” This happens most often in small, tight-knit communities, in which measles can spread like wildfire. 75% of all recent measles cases have happened in those types of communities.

In order for everyone to be protected from measles, all people who can get vaccinated need to. But most states allow parents to opt out of vaccinating their children for any philosophical reason - and that’s allowing measles to make a comeback..."