There is a nice article in the April 2009 article of Scientific American about predicting the next pandemic. I had previously posted to the other flu thread about the 6 phases of a pandemic. This one does a similar thing but makes it a little clearer that the CDC site.

(summarized)
Stage 5 - exists in both animals and humans. Can progress through multiple "generation" of victims in both animals and humans.

Stage 6. "Pathogen has become exclusive to humans."

The article also refers to the HIV virus as being at a "pandemic state".

Wolfe, Nathan (2009). Preventing the Next Pandemic. Scientific American, 300-4. pg. 76 - 81.