The Africanized bee is so defensive that it is also known as the killer bee by many people, largely due to media reports when it goes into attack mode and people get hurt or killed. Genetically, this bee is a crossbreed of the western honeybee, in particular the Italian (A. m. ligustica) and Iberian (A. m. iberiensis) honeybees, and the east African lowland honeybee (A. m. scutellata).
This hybrid bee was bred by biologist Warwick E. Kerr, in Sao Paolo, Brazil, in an attempt to breed a bee that would produce more honey in tropical conditions than the European bees did.
The difference between these two bees is that the western honeybee defends its colony and territory and will sting if it feels threatened, while the Africanized bee is much more aggressive and attacks humans. Africanized bees have killed well over 1,000 people and their victims usually show 10 times more stings than victims of European honeybees. They have also killed horses and other animals.
It can be hard to differentiate between these two bees because they look similar although the Africanized bee is a bit smaller than the European honeybee. Your best way to know is by the behavior. If the bee remains aggressive for up to half an hour or more, it is likely an Africanized bee.
This 3:38-minute video by National Geographic gives good insight into Africanized honeybee facts:
The East African lowland honeybee was initially introduced to Brazil in 1956 to improve honey yields there, but 26 swarms escaped quarantine in 1957. Hybrid versions called Africanized bees started migrating north and the first ones showed up in the USA in 1985 with hives identified in south Texas in 1990. It is considered an invasive species in the US, even though it does contribute to pollination, but due to its temperament it is not a desirable pollinator.
This bee is likely to sting without being provoked, and travels with a large swarm of other Africanized bees. Although a single sting from an Africanized bee is not more painful than a regular honeybee sting, these bees travel in large swarms and sting accordingly. They can chase a person for 400 meters or quarter of a mile. Once targeted by them, a victim could easily receive a lethal dose of venom.
The media sensationalized these deadly Africanized bees in the late 1970s by creating the "killer bees" concept and several horror stories were filmed around the same time. This incited fear in many people and has been at the root of numerous problems for bees ever since. While it is true that Africanized bees are very dangerous compared to more docile European bees, the sensationalistic presentation caused many people to fear all bees, and this has done a great disservice to thousands of bee species ever since.
This is why it is so important for all of us to educate the public about the beauty of all the other honeybee species. When people learn to love bees and respect their space, there are rarely serious problems.