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An attack by Africanised honey bees, known as "killer bees", left one man dead and another critically injured. What makes these insects so deadly?

They surround you in their multitudes - hundreds, maybe thousands of them, swooping and stinging and injecting venom into your flesh.

They might go for your eyes. Or fill up your mouth, nostrils and ears. You will have to run hundreds of yards to escape, if you can see where you are going.

And the air fills with the pheromone that tells the bees to protect their colony. Apparently, it smells like bananas.

On Wednesday morning, thousands of bees surged out of a hive in Douglas, Arizona. Their hive had, according to reports, been left undisturbed in a loft for a decade. These were not the kind of honey bees to which most Europeans are accustomed.

The insects attacked a group of four gardeners who were mowing and weeding for the property's 90-year-old owner. The men, who worked for an organisation that employs people with developmental disabilities, sought refuge.

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But one of the landscapers, a 32-year-old man, suffered cardiac arrest and was later pronounced dead. A witness to the attack said "his face and neck were covered with bees", according to the Arizona Daily Star. One of his colleagues was taken to hospital after being stung more than 100 times.

An estimated 800,000 bees were involved, the local fire department said. While entomologists contacted by the BBC suggested this figure was likely too high, it's clear the colony was huge. The hive filled a 55-gallon (250-litre) drum and part of the building's roof had to be removed to reach it.

It's thought the creatures involved were Africanised honey bees, widely known as killer bees, which have spread across much of the southern United States since 1990.

The sub-species has been blamed for a series of similar incidents. In December 2013 a swarm of about 30,000 attacked a couple in Texas, stinging one of them around 200 times and killing the pair's miniature horses.

Weeks before, another Texas man died after 40,000 bees surrounded him. The previous March, two park employees in Tampa Bay, Florida, survived after being attacked by about 100,000 bees.

Unlike the European honey bee, this sub-species is especially sensitive to the presence of humans, says Jennifer Fewell, president's professor at the Arizona State University School of Life Sciences.

It doesn't take much for them to be disturbed and for their natural alarm systems to be activated.

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"It's very easy to have thousands and thousands of bees around you," says Fewell.

"You get a cascade. It snowballs very quickly. They often target the eyes and the face. They will go all over the body but they will have places they will attack - dark rather than light areas."

The venom of Africanised honey bees is no more toxic than that of their European cousins. What makes them so dangerous is the sheer number which will attack.

They are more aggressive and more easily agitated than other bees. The banana-scented pheromone they release - the one that tells the rest of the colony to join in the attack - can be detected across a wider radius, says Juliana Rangel, assistant professor of apiculture at Texas A&M University's entomology department

Bees sting defensively. But whereas only 10% of a group of European honey bees might defend their hive, the entire Africanised honey bee colony might participate in the manoeuvre.

"If you disturb a European colony you might get a few stings. If it's an Africanised colony it's a few hundred stings," Rangel says.

There isn't much you can do to fend them off, says Fewell, except "run away as fast as you can".

Even jumping in water is a bad idea. Africanised honey bees will wait for you to come up and take a breath, according to Rangel.

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Africanised honey bees originated in 1956 in Brazil, where African and European sub-species were crossbred to improve their tropical hardiness. But, in what sounds like a plot from a cheap horror film, some escaped from the laboratory a year later and migrated north.

They were first detected in the US in 1985 and the first permanent colonies were identified in Texas in 1990. Their population spread very quickly, although their preference for warm climates has confined them to the southern states of the US.

However, the sobriquet "killer bees" has led to an exaggerated fear of how hazardous the creatures are.

It's reported that according to the University of Illinois' Department of Entomology there are about 40 fatal bee attacks in the US each year, and around two million Americans are thought to have bee allergies.

But bees do not seek out humans to pursue. They only sting in an effort to protect their hives and their queen, say entomologists. Bees gathering pollen or swarming - that is, moving between hives - are not a danger.

"Bees remain the most helpful insect to humans - they pollinate one third of food we eat, they give us honey, they make our diet much more interesting," says Rangel.

From a human perspective, Colony Collapse Disorder - in which bee numbers dwindle, hitting the food chain and threatening agriculture - is a much bigger hazard.

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