Why are measles outbreaks increasing in some developed countries?

Measles used to be a common illness, but now it's making a comeback in some big cities because people aren't getting vaccinated as much.

Imagine your school is like a castle. If most kids wear armor (which is like being vaccinated), the bad guys (like measles) can’t get in easily. But if lots of kids don’t wear armor, the bad guys can sneak through and cause a big mess, that’s like a measles outbreak.

Sometimes, parents decide not to vaccinate their kids, maybe because they heard stories or felt unsure. It's like if your friend skipped wearing armor because they thought the bad guys weren’t real, but then the castle got attacked!

Also, some people get vaccinated later or miss a shot. That’s like getting armor halfway through the battle, it helps, but not as much.

When enough people don’t wear armor (or only wear part of it), the castle becomes weak, and that’s how measles outbreaks happen again, even in places where the illness was almost gone.