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:
All living things are made of cells. In the human body, these highly efficient units are protected by layer upon layer of defense against icky invaders like the cold virus. Shannon Stiles takes a journey into the cell, introducing the microscopic arsenal of weapons and warriors that play a role in the battle for your health.