Killer bees, also known as Africanized honey bees, really shouldn't exist in the Americas at all. They were the result of a crossbreeding experiment between European and African bees performed in the 1950s in Brazil in an attempt to come up with a hybrid bee that could survive the tropical climate. The crossbreeding worked and the hybrid bees tend to produce more honey than European honeybees, but some of the hybrid bees got loose and mated with the local bee population. This produced an extremely aggressive strain of "Africanized" bees that, while having a sting that's no more poisonous than that of your average honeybee, is far more likely to defend its hives by attacking in large swarms. Africanized bees can detect intruders on their territory, such as humans, at longer distances than other bees and will chase those intruders for longer distances too. Africanized bees are like tiny, heat-seeking missiles. When they get on your trail, it's difficult to get them off.
By the early 1990s, the Africanized bees had reached the United States, at first spreading into the desert regions of the Southwest. Since then they've spread even further and, while not quite the threat that movies and novels sometimes make them out to be, nonetheless kill one or two people a year. While their reputation as "killer bees" is more than a little exaggerated -- any bee sting can kill a person who has an allergy to bee stings -- it's still probably wise to avoid any bee hives in the vicinity of your camping grounds, even if the inhabitants look like normal, non-aggressive honeybees.