There are a lot of mixed feelings about honey bees. On one hand, they make delicious honey and help plants have sex; on the other, their stings can hurt and trigger insane allergic reactions. On the the third (wait, where did a third hand come from?), they were bred into aggressive super-bees out for blood.
That is an Africanized bee. No, the Cherrios bee did not put on a bunch of gaudy jewelery and start rapping. See that little stick the bee has? It is going to kill you with that.
It all started when one Brazilian beekeeper tried to make his European honeybees (Apis mellifera ligustica) more viable. To do this, he purchased 26 Tanzanian bee queens (A.m. scutellata). He proceeded to breed them in an attempt to make a new bee that was stronger...faster...and more adaptable to the rain forest climate! (Actually, I have seen mixed things about this; some sources say that the hybridization was intentional at first, others say it occurred after the bees escaped.)
Then, by pure accident (or fate, take your pick), the Tanzanian hybrid bees were released. They mated with European honeybee populations. This was more like crossing a dog and a wolf than, say, two different types of python, so nature could definitely do it.
The fluke worked out fine for a while. The bees did adapt better to Brazil's tropical climate than just plain European bees. They are currently the preferred bee in South and Central America, despite a few unfortunate side effects of hybridization. The hybrid bees did very well.
Wait a sec, if the Amazon already has bees, why bother with these?
The bees took over much of South and Central America before slowly moving their way towards North America. As of 2003, they have slipped their way into North America as well. Geez, we knew these bees were created as a result of science gone horribly wrong, but I doubt the breeder had world conquest on his mind.
One of the side effects of crossing African bees into the European sort made them aggressive (or, rather, defensively-aggressive). Although the African blood made them happier in such a hot, humid climate, they were also more easily agitated. They have the nickname "killer bees" for a reason.
These bees are just...jerks. If anything -any animal, even a human- walks by these things, they are going to get stung. The bees are extremely sensitive to noises of any sort, including the ones that come from vehicles. They are even douchebags to other bees, often killing European queens and effectively taking over the hive.
OH, you meant the bees. Right. We are still not amused.
When irritated, the Africanized bees swarm. They do not have more potent venom than the average honeybee; they just attack en masse, even if you have avoided the hive. The bees are also persistent, following targets for up to a quarter of a mile and stinging incessantly for the whole trip. People have died from massive bee attacks, but they are far more dangerous to the apiculturists that provide honey.
There is no visual warning that these bees are psychotic killing machines. They look pretty darn similar to your every day honeybees. (Only an expert can tell them apart, so if you're a certified entomologist, good for you.) It is only when you suddenly notice a giant cloud of bees coming at you that you realize your mistake.
This could be a hive of normal bees and you would not be able to tell.
The best advice is still to let the bees be, but if you see them suddenly picking up shotguns, run like hell.