A British man who suffered memory problems discovered the cause of it four years later - he had a a rare tapeworm running around in his brain. On a daily basis it moved five centimetres from the right side of his brain to the left, and was 1cm in length.
It left the 50-year-old man in excruciating pain for most of the time.
The rare worm was of a type never before found in the UK but it is believed that it can be caught by eating infected food or via a Chinese medical remedy for sore eyes that includes raw frog.
The team which examined the worm said the victim was from a Chinese background and had lived in the UK for 20 years but visited China often.
Dr Effrossyni Gkrania-Klotsas, study author from the department of infectious disease at Addenbrooke's Hospital in Cambridge, said: "We did not expect to see an infection of this kind in the UK, but global travel means that unfamiliar parasites do sometimes appear."
Writing in the Genome Biology journal, scientists revealed that the patient went to hospital in 2008 suffering from headaches, seizures, altered smell ability, memory flashbacks and problems, plus increasing pain on his right-hand side.
Over the next four years he was tested for diseases including HIV, tuberculosis, lime disease and syphilis after MRI scans showed lesions in his brain.
Scientists at St Thomas' Hospital in London finally diagnosed his affliction after taking a biopsy of his brain.
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The tiny tapeworm, Spirometra erinaceieuropaei, was identified by a team of scientists led by Dr Hayley Bennett, from the Wellcome Trust Sanger Institute, which mapped its genome.
It has only been reported 300 times in the last 60 years, the scientists said. It is rare but most commonly found in South East Asia, China and Japan.
As well as the Chinese medicine risk, it is also believed to be transmitted by eating infected crustaceans from lakes or eating raw meat from reptiles and amphibians, they said.
Here is a close-up of a tapeworm:
The patient is now well and scientists believe that by gene-mapping the worm, new treatments can be found for tapeworm infections.
"For this uncharted group of tapeworms this is the first genome to be sequenced and has allowed us to make some predictions about the likely activity of known drugs," said Dr Matt Berriman, from the Sanger Institute.
"The genome sequence suggests that the parasite is naturally resistant to albendazole - an existing anti-tapeworm drug.
"However, many new drug targets that are being explored for other tapeworms are present in this parasite and could offer future clinical possibilities."