Alexander Litvinenko, a former colonel in the Russian KGB spy agency who accused his former bosses of mass murder, has been fighting for his life in a London hospital after an especially memorable meal at a London restaurant three weeks ago.
"As his condition deteriorated," said Dr. Andres Virchis, a hematologist, "it became clear to us that this was no longer just an episode of gastroenteritis or food poisoning, but that actually something serious had happened to Alexander."
Litvinenko's mysterious lunch partner, it seems, could have spiked his sushi with thallium, a toxic metal used in rat poison.
"He's gravely ill, no doubt about that," said Dr. John Henry, a clinical toxicologist who has seen Alexander Litvinenko in the hospital. "And it is due to thallium … and it's been confirmed in his blood stream."
"I saw him yesterday, he looks terrible," said Alexander Goldfarb, a friend. "He looks like a ghost, actually. He lost all his hair. He hasn't eaten for 18 days. They keep him on i.v., and he looks like an old man."
British police now have security camera pictures from the restaurant, but have made no arrests.
Still, Litvinenko's friends claim it was payback for his attempts to poke around in the dark corners of the Russian intelligence services.
One of those pals is Russian dissident and tycoon Boris Berezovsky, who has also been at Litvinenko's bedside. Litvinenko had previously accused Russian agents of trying to murder Berezovsky after he had a falling out with Russian President Vladimir Putin.
The Kremlin has not commented on the new alleged poison plot. But Berezovsky said his friend has been a victim of a conspiracy at the top.
"It's not complicated to say who fights against him," Berezovsky said in a telephone interview. "He's Putin's enemy. He started to criticize him and had lots of fears."
Litvinenko wrote a book in which he accused Russia's secret service of staging a series of Moscow bombings in 1999 that killed 300 Russians, and further accused agents of blaming the deaths on Chechen rebels in order to gain Russian public support for war against Chechnya.
Goldfarb, the friend at the hospital who arranged Litvinenko's 2000 emigration to Britain, said FSB [Russian security] agents had made death threats against Litvinenko.
Litvinenko recently said he was investigating the murder of Russian journalist Anna Politkovskaya, a severe critic of Russian tactics in Chechnya, and he was enticed by an offer to solve her murder case.
"Someone came up to me and said that we should meet," Litvinenko recently told the BBC. "The meeting took place in a restaurant in London. He gave me some papers which contained some names, perhaps names of those who may have been involved in the murder of Anna Politkovskaya. And several hours after the meeting, I started to feel sick."
In an interview given to The Sunday Times newspaper before his condition worsened, Litvinenko described how he had lunch with the Italian contact who claimed to have information on Politkovskaya's killing.
"They probably thought I would be dead from heart failure by the third day," the newspaper quoted Litvinenko as saying. "I do feel very bad. I've never felt like this before -- like my life is hanging on the ropes."
Poison nearly changed the course of history when Viktor Yushchenko, the youthful President of Ukraine, suddenly aged overnight and nearly died in 2004 when he was given a secret dose of dioxin during the election campaign.
And in London in 1978, Georgi Markov, a pro-western dissident in Bulgaria, was stabbed and killed with a poison-tipped umbrella.
As for the new alleged poison plot, a hospital statement said Litvinenko is in serious but stable condition.
British police are saying nothing. Russian officials are saying nothing.
But somewhere out there, someone apparently still knows how to use an old Cold War weapon -- and is saying nothing.
ABC News' Joe Simonetti and Roger Kaplinsky-Dwarika contributed to this report.