Nearly 700 measles cases have been reported in the US this year, with the bulk of cases in New York and Washington states. That's the highest number of US cases in 25 years.

The measles vaccine was developed in the early 1960s, and measles was declared eliminated from the US in 2000.

When there are enough vaccinated people in a population, the measles virus doesn't have a chance to spread. But experts worry that as more kids forgo vaccines, the virus could grab a "foothold" in the country once again.

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The measles is back. The extremely contagious virus is circulating once again across the US, especially in pockets of the country where unvaccinated people are clustered and at risk.

With nearly 700 cases reported so far this year — the highest total in 25 years — the US Centers for Disease Control and Prevention (CDC) is warning that "the longer these outbreaks continue, the greater the chance measles will again get a sustained foothold in the United States."

Before the invention of the measles vaccine in the early 1960s, the measles killed 400 to 500 people in the US every year. Public health experts are now concerned that the illness could turn deadly once again (though no US deaths have been reported so far this year).

Here's a look back at what the measles really looks like when people are not vaccinated.