ELI5: Why do people who never met each other sometimes catch measles from the same place?

Imagine you're at a playground on Saturday. Lots of kids are playing!

Now, imagine a kid, let's call him Alex, has the measles. Alex coughs, and tiny, tiny bits of the measles virus float in the air. Because the playground is a big, open space, the virus bits spread around a little. Alex doesn't stay long and goes home.

Later that day, another kid, let's call her Bella, comes to the same playground. Bella breathes in the air, and some of those measles virus bits get into her body. Bella hasn't had the measles vaccine, so her body isn't prepared to fight the virus.

A few days later, Bella starts to feel sick and gets the measles!

Even though Alex and Bella never saw each other, Bella caught measles from the same playground where Alex spread the virus.

Here's why this happens:

• Measles is super contagious: It spreads very easily through the air when someone who is sick coughs or sneezes.
• The virus can linger: The measles virus can stay in the air or on surfaces for up to two hours.
• Not everyone is immune: If someone hasn't had the measles vaccine or already had the measles, they can catch it.
• Shared Space: The playground is the common place where the virus was transmitted. It's like a chain reaction, but it started at the same spot.