Because of this year’s sharp increase in measles cases — which the Centers for Disease Control and Prevention has blamed a deliberate misinformation campaign by anti-vaccine activists for — many Americans are wondering whether they need to be vaccinated.

The C.D.C. emphasizes that children are the most important group to reach. Outbreaks spread rapidly in preschools and kindergartens, and young children often have infant siblings too young to get the vaccine.

But some adults, too, should consult with their doctors and consider getting the shot.

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Who is immune to measles?

The vast majority of adults in this country are immune, according to the C.D.C. and other infectious disease experts. If adult Americans were not immune, the virus — which has been introduced from overseas every year since endemic circulation was eliminated in 2000 — would have spread widely before now.