Cells have had many fights over millions of years...
​And one of it's least favorite people are Viruses. Viruses enter the body through any opening and they make you go Ill.
​Viruses are super small... like really super tiny.

As you can see the virus shown in the picture is smaller than the host cell.
​A viruses goal is to get to a cells nucleus if it gets there. It can rewire the cell. Make it so the nucleus stops making the cell parts. And it makes virus construction parts instead... Uh oh. Later the cell dies and millions od new viruses leave the dead cell to infect more...
The immune system is called in...
 Immune Force.
​ B cell, Dendritic Cells, Granulocytes, T cells, Memory Cells, ILCs, Megakaryocytes, Natural Killer Cells, Macrophages, Monocytes. And Some more.

​Here is some videos that helped me memorize The immune system and making it my favorite subject in science.