It has been the cough which has dogged Hillary Clinton's campaign - and hours after it failed to surface at the presidential debate, it was back.

Clinton covered her mouth briefly to cough as she spoke on Tuesday to reporters on board her plane.

It interrupted a claim of triumph in the debate, which many had been watching avidly for clues as to her true state of health.

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Returned: Clinton went 95 minutes without coughing on Monday night but it resurfaced in a brief conversation on Tuesday morning on her campaign plane

Holding on: Clinton took a firm grip of the rail as she climbed the steps to her campaign's Boeing 737 at White Plains Airport, near her Chappaqua, NY, home

But she was unaffected by either the cough or the other issues which have been commented on during her campaign - particularly her eyes not moving together.

Some observers, including conservative radio host Hugh Hewitt, had suggested that one of the major elements of pressure on Clinton on Monday night was to avoid coughing.

He had told MSNBC: 'She has so many things she can't do. She cannot cough, she cannot laugh with the distinctive Hillary laugh, she cannot shout like she did at her supporters last week and she cannot condescend.'

Clinton successfully lasted the full 95 minutes of the event - which over-ran by five minutes - and it was Trump's persistent sniffing which was widely noticed.

Her health was not raised by Lester Holt, the debate moderator, and Trump also refrained from directly discussing it.

On the trail: Clinton was back on the cmapaign trail with a flight to North Carolina, a swing state her campaign believes she can win

Ginger steps: Clinton exited her campaign plane in the rain and held on carefully to the rail

Instead he raised it after Holt asked him about a previous comment, that Clinton did not have 'the presidential look'.

'She doesn't have the look. She he doesn't have the stamina,' he said.

'You have so many different things you have to be able to do and I don't believe Hillary has the stamina.'

Clinton stood stock still, waiting to pounce.

First, she let fly a recitation of her exploits as secretary of state: travels to 112 countries, negotiations on peace deals, cease-fires and imprisoned dissidents — even the 11 hours she spent testifying before a congressional committee investigating the Benghazi situation.

Once Trump can do all that, said Clinton, 'He can talk to me about stamina.'

The health questions, however, are unlikely to go away.

Clinton's collapse at the New York 9/11 memorial service led to her finally revealing a diagnosis of pneumonia, and that she had had surgery in January without it being disclosed.

Her eye movements have also raised questions, with her two eyeballs moving in apparently different directions.