End of the first movie the pilot (who Ceasar had bit) had his nose bleeding as he got on a flight. They showed a flight board with cities all over the world. The infection killed billions and this movie is about the few humans that remained trying to get energy from a Dam in ape controlled territory.
but how did the ape population get so large?
Who knew that there were so many apes in the US?
Actually that was a scientist who worked with Caesars friend at the drug company. He was exposed to the virus that gives apes intelligence. apparently its a deadly pathogen to us.