In Plague Inc. for the Android & iOS (iPhone, iPad, iPod Touch, etc.), there are several different plague types to play with, but only Bacteria is unlocked from the beginning.
The plagues are unlocked in order as you beat each type, so to unlock the Virus type, you just need to win your first game against Bacteria.
I wrote up a long article on beating Bacteria on Normal if you’re having trouble. This should get you going on how to form strategies a lot faster.