Common problem - bee swarms can be a serious problem during summer - most often when commercial honey bees swarm to establish new hive inside a wall cavity of a building. 

Extremely aggressive - a swarm of bees can be extremely aggressive and may attack a person in such large numbers as to cause death. The venom of bees is highly toxic and may cause hideous swelling of the skin, particularly of children and people with a fair complexion. Some people are highly allergic to the venom of bees and deaths are reported to occur in Australia every year as a result.

Allergic reaction - in some cases, death may arise to people who are allergic to the bee venom or who are stung by the swarm of bees in large numbers. 

DO NOT attempt to eradicate a group of swarming bees, unless you have the complete range of protective equipment and professional knowledge essential in the circumstances.