A health crisis is when lots of people get sick or hurt at the same time, and it starts to become a very big problem for a lot of people. Think of it like when you and all of your friends get the flu at the same time and everyone has to stay home from school for a while. That's a small health crisis.
But sometimes, a health crisis can happen on a much bigger scale. Like when there's a new disease that spreads really quickly and makes a lot of people sick all over the world. Or when there's a natural disaster like a hurricane or earthquake that destroys houses and hospitals, making it hard for people to get the medical help they need.
When a health crisis happens, doctors, nurses, and other medical professionals work really hard to find ways to help all the sick people. Sometimes they have to work extra long hours or use special equipment to keep everyone safe. And all of us can help too by washing our hands, wearing a mask, and staying away from crowded places to slow down the spread of sickness.