Pandemic2

In this game, Pandemic2, you have one goal: reach the highest level of virulence possible. You can start off with either a living pathogen (parasite or bacteria) or go with a non-living pathogen (virus) to start planning the massive destruction of society. Throughout the game you will get points that you can increase your modes of transmission to make transmissability easier.  You can also increase your resistance to various factors, like water, heat, or antibiotics, while adding to the symptoms and lethality of the pathogen. Warning: this game is pretty addicting. See if you can find the best combination of symptoms, resistance, and modes of trasmission to infect everyone in the world!

http://www.addictinggames.com/strategy-games/pandemic2.jsp