alt Hacker NewsPlague, Inc (an iOS game where you control the parameters of a pandemic and try to get a 100% infection rate) will give you a really good feel for the math behind this.
The most successful strategy is to make a virus that spreads fast, with few visible symptoms until the late stages of the disease. A deadly virus, early will just cause borders to be locked and the international research community to swarm on a cure.
If AIDS were airborne, I think we'de have a fraction of the billions currently living today. It takes a while for symptoms to show up, there is still no real cure, and drugs to keep it supressed took many years to develop.