Press the green flag and then click the buttons :).
So in case you don't have any idea what's going on with the animation I'll tell you. The virus attacks the cell by attaching it's protein fiber coat so then the cell will let the virus get in. Then the virus inserts it's DNA, making the cell replicate it's genetic material. Then it will put all the viruses together making thousands of viruses and will burst out of the cell destroying it, going on to infect other cells.