You are the monster; a deadly green vomiting virus infecting the stomachs of your victims. Float your way through stomachs infecting red cells as you go, avoid the white cell or take them down if you are big enough, and watch out for the yellow cells. Infect everything and your dying victim will empty their stomach spreading the virus.
To move, use the mouse to control the direction and speed of the virus.
That's it!
The first few victims have special "tutorial" stomachs complete with instructions.