It's your job to infect everyone in the world with your disease. Your virus starts out in one country and you have to spread it around to other countries, but these countries will shut down their borders if they start seeing a virus developing in another country. Timekiller #1.
Basically, the louder the "cow" gets, the closer you are to him. When he is really loud, you are close, and maybe on him. It seems simple but gets really addicting.