They die over time but new ones take its place...
For Example: Pretend that your sick and your cells are soilders. They would call in reinforcements and rick their lives to keep you healthy. They die once the battle is over. I call it the "Virus".
Some cells self destruct to save energy in your body. When you sneeze your technically desposing the self destructed cell. The cell that self destructed in your body turns into wet..powder.....that's what it looks like atleast.
I hope this helped :P