A schoolgirl spent a week in hospital after being bitten by a venomous false widow spider.
Layla Benton, 14, had to be treated for blood poisoning and cellulitis and doctors had to contact a tropical diseases centre for help.
Her mum, Wendy, from Partridge Green, Pitsea, Essex, said: "Layla came down from the bathroom and told me she had been bitten by something and her knee was stinging and itchy.
"Within a few hours it really started to react. She had antibiotics from our GP, but it didn't do any good and she had to be admitted to Basildon Hospital.
"It got really bad. She developed cellulitis and blood poisoning. My other daughter found the spider in the bathroom and we took it to the hospital - but they just laughed and said it had been in the country for years, and it was the way Layla had reacted to the bite.
"They contacted a tropical diseases hospital in London and had some special cream couriered down, but that made it worse and had to be washed off.
"It was awful by then, with vivid orange stuff leaching from the wound. At that point they said she might need to have surgery to wash it out."
Layla spent three weeks off school. She will have a small dent in her knee where the poison damaged the flesh.
The false widow spider, Steatoda nobilis, is about the size of a 50p coin. No one is known to have died from its bite, but some people have extreme reactions and can lose limbs as a result.
Symptoms can include severe swelling, chest pains and tingling of fingers.
Earlier this month Ricki Whitmore, 39, from Collier Row, Essex, almost lost a leg after he was bitten, and John Catlin, 66, from Bromley, almost had his foot amputated after a bite on the toe.
But experts say the species is not aggressive and unlikely to bite unless threatened or disturbed, such as when prodded or accidentally squashed.
They say a changing climate - with a wet start to the summer followed by a heatwave - could be responsible for and increase in numbers, with an estimated 10 million false widows in the UK.