I have a wee bit more tweaking to do with the game play, but I think I am almost ready to submit my entry... Contamination.

https://www.youtube.com/watch?v=B2W8GJ-vz0I

In the lab is a petri dish with an experiment. Protect the green cells from the red germs by shooting them. The scientist who is watching is coughing and sneezing on the petri dish, and this is causing more germs to invade the experiment. Look out for the Super Cell... you can pick that up and use it to wipe out all germs in the dish.