Staying healthy is a lot like medieval warfare. Cells vs viruses. There are cells defending their castles and viruses trying to break through. If a sneaky virus manages to attack a cell, the cell fights it and notifies all the other castles about what to build to defend it.
Man, learning about biology is so much easier when you have cute animations like this making it look like Game of Thrones.
The animation was done by Igor Coric and the lesson was made by Shannon Stiles. Ted-Ed explains: