He was the dream athlete of the London Olympics but when he returns to the Stadium on Friday night Usain Bolt is obliged to fight off rather more than the threat of Britain’s sensationally quickening James Dasaolu.
Bolt has become track and field's most significant defence against the old seeping nightmare that was confirmed most dramatically with the disgrace of Ben Johnson in Seoul in 1988.
Ever since his compelling triumphs in the Beijing Games of 2008, Bolt has argued that his success has come not from an illicit bottle or syringe but extraordinary gifts of nature.
That was again precisely his rebuttal of the doubts that dominated today's build up to the Anniversary Games that were supposed to be a joyous celebration of the dazzling events that captured the imagination of the nation and the world last summer. Bolt, who won three gold medals at the Olympic Stadium capped by a withering anchor run in the men's relay, pointed out he had been performing “phenomenal” deeds since he was 15. He didn't have the profile of a doper, just the inexorable progress of a natural-born winner equipped not with glowing musculature but a gangling gait that simply ate up the ground.
Beside him 25-year-old South Londoner Dasaolu - just a year Bolt's junior - had explained that his eruption as one of the fastest men in the world at such a relatively late stage of his career was to do with much better medical supervision.
In the past his most striking points of achievement had been accompanied by physical breakdown. Now more closely monitored, he was able to muster the kind of performance which brought a huge leap forward at the recent British trials with a personal best of 9.91sec, the fastest time by a home runner since Olympic gold medallist Linford Christie ran 9.87 in 1993.
Dasaolu today argued that although the recent spate of high-profile positive tests was a setback for athletics it was necessary to focus on brighter aspects of the sport - and notably the drive to educate British athletes.
It was a solemn and perhaps worthy call but maybe not one guaranteed an overwhelming response when you considered the latest surge of doubts.
When Dasaolu ran his superb time it took him to fourth place in the year's 100 metres rankings - behind Tyson Gay, Asafa Powell and Justin Gatlin - and one place ahead of a Bolt who today admitted that he had still to find some of the best of his form.
You didn't have to be cynical - or easily dismayed - to wince at that particular league table. Gay and Bolt's Jamaican team-mate Powell fell in the recent spate of failed tests, along with four other members of the Jamaican team. Earlier in his career, Gatlin was handed a four-year suspension after testing positive. He won Olympic gold and silver in Athens in 2004, then in London added silver and bronze.
These are the kind of obdurate realities which mean that the thousands who go to the Olympic Stadium tonight are again required to walk a most precarious line between belief and doubt. In London last year Bolt invaded the doubt with glorious performances and the gifts of a vaudevillian. His weapons on behalf of belief were brilliant, knock-about humour and performance that made you ache to accept it was a purely natural force. Watching him win was rather like watching the surf roll in.
Afterwards, Bolt was angry in the way of Sir Bradley Wiggins and his successor in the yellow jersey of the Tour de France, Chris Froome, when the spectre of drugs was raised. He swore that he was clean - and rejected the idea that to express doubt was merely to react to the overwhelming force of track and field history.
Today he was somewhat more temperate, after suggesting that it was a little early to discuss the latest positive test - on a Jamaican World Cup footballer.
“I'm clean,” he declared, “but you have to be very careful as an athlete because right now there are a lot of things on the banned list. For me I get tested all the while. I got tested the day before yesterday. It's just part of the routine. You might not know and you have to keep up to date with this kind of thing. It's kind of hard but that's why you have a team to make sure.”
Such certainty will seem as remote as ever as those who want it so badly return to the stadium in Stratford where Bolt for a few charmed and exhilarating nights repeated his achievements in Beijing.
What he did was suspend the doubt, produce performances that briefly stopped the world. In Seoul, when Johnson raced to the gold that was so swiftly stripped away, the abiding memory was the shocked expression on the face of the great, if not totally unsuspected, Carl Lewis, when he saw the world-record-shattering time flashing up in lights.
In Beijing there was a quite different reaction. It was one which was perhaps best categorised as a huge benefit of the doubt. Bolt ran so uniquely, so irresistibly, it seemed like the best of reasons to believe that he had indeed done it cleanly. In Seoul there was a gasp when the world looked at the flashing time. In Beijing there was a great roar and in London the thunder returned.
Tomorrow night it will be heard again but maybe not so loud. The glory, it is hard not to believe, has rarely been so fragile.