By Steve Holland and Emily Stephenson

WASHINGTON (Reuters) - Hillary Clinton's bout of pneumonia has shed light on a problem seldom seen by American voters: The long days, little sleep, cross-country travel, bad food and kissing babies add up to a recipe for illness for presidential candidates and aides.

Avoiding viruses and other ailments can be next to impossible for people who spend months in the close confines of campaign planes and buses.

Brooke Buchanan, former press secretary to 2008 Republican presidential candidate Senator John McCain, remembers leaving the campaign trail in Beaufort, South Carolina to visit an emergency room. She had a respiratory ailment and two ear infections.

"You have to soldier on during certain things, but there's a point when you become a liability," Buchanan said. "I was back on the road the next day, full of antibiotics."

Supporters of Clinton, who will face Republican Donald Trump in the Nov. 8 election, worried on Monday that the Democratic presidential nominee's medical scare would fuel conspiracy theories about her health. But Republican and Democratic political veterans alike say illnesses are an unwelcome but standard part of life on the campaign trail.

Alice Stewart, who was a senior adviser to former presidential candidate Senator Ted Cruz during the Republican primary earlier this year, remembers coughing her way through some of his news conferences while she was recording them for the campaign. She called the Cruz plane a "flying petri dish."

"You just kind of power through it. When you're on the road, you can't drive home," Stewart said.

In recent weeks, several staff at Clinton's campaign's Brooklyn headquarters have fallen ill and required medical treatment, a campaign aide said.

Steven Simpson, a pulmonary specialist at the University of Kansas, said candidates were particularly vulnerable to illness.

“The average patient, if you have the luxury of it, takes the week off before you go back to your full duties,” Simpson said. “But how do you say that to a presidential candidate?”

OBAMA'S HANDKERCHIEF

President Barack Obama caught a cold in August 2008, shortly before the Democratic Party's convention. A video posted on YouTube shows him sneezing at a rally. "That's why I've got my handkerchief," he said.

"Every candidate and campaign aide gets sick, but they just never get to take days off," said former Obama aide Dan Pfeiffer. "You take vitamin C and try to get sleep, but it's a losing battle."

Obama's 2008 opponent, John McCain, traveled at times with a friend who was a doctor, said Steve Schmidt, the Republican candidate's campaign manager.

"I think one of the secrets of working in a White House or on a presidential campaign is it's the closest you can ever get to traveling with Elvis (Presley) when it comes to the number of pills that are floating around," Schmidt said.

"There was always a big bag of every conceivable type of antibiotic and cold medicine, and you name it - we had it."

Republican presidential nominee Mitt Romney developed a respiratory illness days before delivering his acceptance speech at the Republican National Convention in Tampa in 2012.

"I was scared to death that he wouldn't really be able to speak," said Stuart Stevens, who was Romney's senior adviser. "Doctors tell you to rest. But there's no time for that."

The grueling schedule led Stevens himself to develop pneumonia right after the 2012 campaign ended.

CANDY BARS AND SAUSAGES

Unhealthy food is another hazard of the campaign trail. Junk food is ubiquitous on campaign planes, and candidates flock to gatherings such as the Iowa State Fair that serve up fried candy bars, sausages and other high-calorie fare.

Romney, who also ran for president in 2008, would sometimes pick the cheese off his pizza and the skin off fried chicken to limit the fat content, said former spokesman Ryan Williams.

"It’s not healthy food, it’s campaign food," said Williams. "And most operatives see themselves put on 10 or 20 pounds by the end of the campaign."

Trump is a noted germophobe, and there has been little evidence of him having gotten ill. He is a proud consumer of fast food, once tweeting a photo of himself on his plane with a bucket of Kentucky Fried Chicken nearby.

That was also the chicken of choice for 1996 Republican nominee Bob Dole, who managed to avoid getting sick throughout his campaign, said his then-traveling press secretary, Nelson Warfield.

"Much of the campaign staff that was half his age were hobbled at some point or another during the campaign, but Dole was resilient," he said.

Still, Dole had one nasty stumble. He fell off a stage in Chico, California, at campaign rally on Oct. 18, 1996. He got up smiling and soldiered on.

(Reporting by Steve Holland and Emily Stephenson, additional reporting by Alana Wise in Washington and Amanda Becker in New York, editing by Caren Bohan and Ross Colvin)