You must protect your heart in the centre of the screen from viruses travelling towards it. Press buttons around the centre of the phone (1-9 except 5) to fire antibodies in the correct direction.  You can only fire when fully charged.
To kill a virus the color of the antibody must match the colour of the virus. Press 5 to change colour.
Larger, slower viruses will only be destroyed if hit by three antibodies of the correct colour.