Africanized honeybees are called "killer bees" because they aggressively attack in great numbers, leading to massive stinging incidents. The large amount of venom injected by many bees at one time can cause death.
The venom of the Africanized honeybee is no more likely to cause an allergic reaction or toxic reaction than that of the European honeybee. But the European honeybee is not as aggressive.
Africanized honeybees were introduced into Brazil from Africa in 1956 and have gradually spread north. These bees entered the United States in south Texas and are now present in Arizona and California. The bees are expected to continue moving northward. But they do not survive well in cold areas.