The speculation is over. He was poisoned.
Medical experts have confirmed that Viktor Yushchenko, Ukraine’s opposition leader, was poisoned in an attempt on his life during election campaigning, the doctor who supervised his treatment at an Austrian clinic said yesterday. [...] John Henry, a prominent British toxicologist, has suggested that Mr Yushchenko’s symptoms were consistent with dioxin poisoning, which causes a severe form of acne called chloracne.
Unfortunately, dioxin doesn't just go away -- just ask all the guys who were exposed to Agent Orange in Vietnam.