Mr Yushchenko says poison caused a sudden change in his appearance
Ukrainian opposition leader Viktor Yushchenko first claimed that he had been poisoned in September, when he was admitted to a clinic suffering from stomach pains.
It was only slowly that his face began to change, a mask of lesions and blisters disfiguring the 50-year-old's previously youthful looks.
The Austrian doctors who treated him initially said they could not confirm the cause of the illness.
Food poisoning was the first diagnosis made by Ukrainian doctors on 6 September.
Mr Yushchenko's political opponents suggested he had eaten bad sushi, washed down with too much cognac.
One rival presidential candidate said he always stuck to more patriotic food, such as pork fat and vodka.
And a top aide to President Leonid Kuchma suggested that other members of Mr Yushchenko's team should test his food.
He too recommended 100 grams of vodka, if problems continued.
But it also transpired that shortly before becoming ill, Mr Yushchenko had had dinner with the chairman of the Ukrainian Security Service (SBU), Ihor Smeshko, and his deputy, Volodymyr Satsiuk.
Mr Yushchenko himself had called for the meeting to discuss the security services' role in the election campaign.
MYSTERY ILLNESS TIMELINE
5 September: Yushchenko has dinner with Security Service chief
6 September: Yushchenko falls ill, Ukrainian doctors diagnose food poisoning
10 September: He is taken to Rudolfinerhaus clinic in Vienna
21 September: Yushchenko tells parliament he has fallen victim to "Ukraine's political cuisine which kills"
30 September: Returns to Vienna clinic
31 October: First round of presidential election
21 November: Second round of presidential election
11 December: Austrian doctors back poison theory
Mr Smeshko made no secret of the dinner when he spoke to journalists on 28 September, after reporting to a parliamentary commission of inquiry.
He later said it was a "matter of honour" for the SBU to investigate the poisoning allegations.
For Mr Yushchenko's supporters, the fact that their candidate fell ill after dining with the security chiefs appeared to confirm the poisoning theory.
Furthermore, after his first consultation with Ukrainian doctors on 6 September, Mr Yushchenko's health failed to improve.
On 10 September, he was sent to Vienna to consult Austrian doctors at the Rudolfinerhaus clinic.
Eleven doctors diagnosed his illness as "acute pancreatitis accompanied by interstitial edematous changes"
His liver, pancreas and intestines were swollen and his digestive tract covered in ulcers.
Mr Yushchenko went back on the campaign trail after a week, against the doctors' advice.
He spoke to the Ukrainian parliament where he accused the authorities of organising an attempt on his life, saying he had fallen victim to "Ukraine's political cuisine which kills".
Campaign officials working for Mr Yushchenko's main rival, Prime Minister Viktor Yanukovych, denied the allegations.
After two weeks, Mr Yushchenko returned to Vienna with back pain, but again he did not stay long.
He hit the campaign trail with his face half paralysed, reportedly with a catheter inserted in his back so that painkillers could be injected into his spine.
Medical experts were divided over the cause of the mystery illness.
Professor John Henry, a toxicologist at St Mary's Hospital in Paddington, London, was one of the first to say Mr Yushchenko had been poisoned with dioxins.
He said the marks on Mr Yushchenko were chloracne, an acute form of acne which can be caused by dioxins.
Professor Henry said the dioxins, which are a by-product of many industrial processes and can increase the risk of cancer and cause infertility, could have been hidden in food or drink.
However, others are not so sure.
Nick Edwards, a clinical scientist at Guy's and St Thomas' Hospital in London, said: "Chloracne is a side effect of dioxin exposure but I have not heard of dioxins being used as a malicious chemical agent in this way."
Dr Stephen Mandy, professor of dermatology at the University of Miami in the US, suggested the marks may be rosacea, an inflammatory skin condition which can leave the face swollen and lumpy.
Dr Mandy told the Associated Press news agency: "Rosacea can explode under heavy stress and this looks like he has a typically fulminant case."
A British poisons expert, who did not want to be named because of the controversy surrounding the election, also played down the dioxins poisoning theory.
"You would need huge amounts of dioxins to poison someone, I just don't believe it.
"The marks on his face could have been caused by stress, that is much more likely."
The medical team at the Rudolfinerhaus Hospital in Vienna remained cautious, declining to speculate about the cause of the illness until they had conferred with experts overseas.
Details of the case were sent to experts in the US, who examined blood and other samples.
More than three months after he first arrived in Vienna, the clinic concluded that Mr Yushchenko had indeed been poisoned with dioxins.