Hillary Clinton allowed her staff to mistakenly think that she had a pulled muscle rather than tell them she had a blood clot in 1998.

The Democratic Presidential candidate, who was First Lady at the time, did not correct White House staffers when they assumed the problem with her right leg was due to over exercising.

According to Clinton's doctor at the time, Connie Mariano, 'very few' people knew the truth and her boss refused to take time of the campaign trail for her husband's re-election.

Instead a nurse came with her to check on her condition and administer drugs if needed but nobody outside of her Secret Service detail was told who she was.

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The collapse: Clinton's astonishing collapse as she left the New York 9/11 memorial service was captured on video and led to the admission she had pneumonia and kept it secret

On the trail: Hillary Clinton campaigned heavily for Democrats in the 1998 Senate elections, including Chuck Schumer, who went on to defeat the Republican incumbent

The incident shows that Clinton's habit of choosing secrecy over coming clean has a long history.

It would not be until 2003, five years after the blood clot and two years after her husband left office, that Clinton would even disclose it had happened in single paragraph in her memoir.

Clinton's health has become an issue during the Presidential election after she nearly collapsed during the 9/11 memorial event at Ground Zero in New York on Sunday.

A video shot by a bystander shows the 68-year-old's legs buckling as tries to get into a waiting van.

Her campaign team said she was 'overheated' but later that on Sunday Clinton's personal doctor revealed she had been diagnosed with pneumonia two days before - but she kept it a secret.

During her decades in public life Clinton has suffered a number of health scares and her response to handling them appears to have changed little.

Rear Admiral (lower half) Connie Mariano. She revealed Clinton's health secrecy in her memoirs

The first of her three blood clots happened in 1998 whilst she was campaigning for her husband Bill to get his second term as President in the face of possible impeachment over the Lewinsky scandal.

According to Mariano, who as a Navy captain and latterly rear admiral (lower half), was the White House doctor for President Clinton, she told his wife that she was 'at risk' for a blood clot and insisted on coming to see her on a Saturday.

In her memoir The White House Doctor: My Patients Were Presidents, she takes two pages to explain how she told the First Lady: 'I'm worried about your leg. You've been out on the campaign trail for some months now, sitting in cars for hours at a time.'

Within an hour Mariano was examining Clinton's right calf which was 'swollen'.

Marian's initial examination caused her enough worry to insist Clinton go to Bethesda Naval Hospital for an ultrasound.

Once they were there and the diagnosis confirmed Mrs Clinton 'balked' at the idea of being kept into hospital so they agreed to treat her as an outpatient with blood thinning drugs.

The book says: 'Hillary was back on the campaign trail within days...very few people knew of Hillary's blood clot at the time; she wrote about it after she left the White House.

'She did have a pain in her calf but her staff thought she had pulled a muscle exercising.

'If my slogan was: "Don't let them see you sweat", Hillary's must have been: "Don't let them see the pain".'

Mariano wrote: 'I assigned a female nurse in civilian attire to accompany Hillary on the campaign trail.

'I also notified the Secret Service that she was on medication. In the event she was injured, excessive bleeding would be a dangerous complication.'

Cordon sanitaire: Massive barricades have been set up around the Clintons' Chappaqua, New York, home while she recovers from pneumonia

Clinton first revealed the blood clot in 'Living History', her 2003 memoir, though with much more brevity.

She devoted just one paragraph to the incident and said that it happened in New York when she was attending a fundraiser with Democratic Senator Chuck Schumer.

Clinton wrote that her right foot was 'so swollen I could barely get my shoe on'.

The book says that Mariano 'whisked me off to Bethesda Naval Hospital' to determine if she had a blood clot from all the 'nonstop flying around the country'.

Clinton wrote: 'Sure enough I had a big clot behind my right knee that required immediate treatment.

'Dr Mariano told me to stay in bed and take blood thinners for at least a week. Although I wanted to take care of myself, I was determined not to cancel any of my campaign stops.

'So we compromised. She sent a nurse to administer the medication I needed and to monitor my condition.'

In her book Clinton did not mention the secrecy around her condition.

Among the other health problems Clinton has suffered was a fractured elbow in 2009 while she was walking to her car at the State Department - at the time she was Secretary of State.

A report by the New York Times from 2009 sought to cast Clinton's illness in heroic terms and claimed that as she was lying on the ground she continued to issue orders to Richard Holbrooke, the White House special representative for Afghanistan and Pakistan.

Despite the pain from her injury Clinton supposedly told him: 'Go to the meeting, that's an order'.

On the stump: Hillary Clinton made a massive effort to boost Democratic chances in the 1998 Senate elections - at the expense of her own health

Such stubbornness and refusal to take orders from others was reportedly the reason why she got dehydrated at the 9/11 memorial.

According to Politico Clinton has clashed with her aides who ask her to drink more water.

As one put it: 'She won't drink water, and you try telling Hillary Clinton she has to drink water'.

Clinton's most serious health episode was in 2012 when she suffered a concussion having fainted due to dehydration and hit her head.

Clinton delayed an appearance before Congress to testify about the Benghazi terror attack and took six months to fully recover.

She was put on blood thinning drugs which she still takes today, as well as a common thyroid hormone-replacement pill.

Last year the Clinton campaign released a letter written by Dr Lisa Bardack, Clinton's personal physician, which said that she had fully recovered from the concussion and was healthy.

This year while campaigning for the Presidency Clinton has suffered numerous coughing fits.

The most serious was on September 5 at a rally in Cleveland, Ohio, where she had to pause and drink some water; it appears she was already suffering from pneumonia by that point.

WHAT WE KNOW ABOUT CLINTON'S HEALTH - AND WHAT WE DON'T

Clinton's medical history – and her tendency to keep personal and medical information far from view – is coming in for new scrutiny following revelations that the candidate got diagnosed with pneumonia Friday

Hillary's dramatic collapse in New York on Sunday is prompting new examination of her health – as well as how her aides have closely guarded information

1998 Blood Clot

Clinton's first known blood clot occurred in 1998, while she was still first lady.

Clinton experienced symptoms while attending a fundraiser for Sen. Charles Schumer of New York, who would soon become her Senate home-state colleague. Her right foot swelled up to the point where she couldn't put on her shoe.

Clinton got quietly taken to the National Naval Medical Center in Bethesda for treatment at the time. She was found to have 'a big clot' blood clot behind her knee, Clinton wrote in her memoir, 'Living History.'

She called it 'the most significant health scare I've ever had,' the Washington Post noted.

According to her physician, Mt. Kisco physician, Lisa Bardack, Clinton was advised at the time to take Lovenox, described as a short-acting blood thinner, when she took flights. The meds were discontinued when she went on Coumadin.

2009 Blood Clot

Clinton had a second blood clot incident in 2009. The episode was described by her doctor in a 2015 letter.

The doctor didn't provide a detailed description of the event. Rather, she wrote that Clinton's 'past medical history is notable for a deep vein thrombosis in 1998, 2009 and a concussion in 2012.

Clinton takes a daily blood thinning medication for her deep vein thrombosis.

Clinton, a frequent flier whose staff catalogued her pursuit of the overall mileage record as secretary of state, may have exacerbated the problem through her extensive air travel.

Frequent jet travel can exacerbate blood clots, which is why some people make sure to walk around the cabin on long flights

2009 Elbow Fracture

Clinton had to work from home for a while after she fractured her elbow during a fall in 2009, CNN recounted. She fell at the State Department on the way to the White House, and went to George Washington University hospital for treatment.

She underwent a two-hour surgical procedure.

'She is working from home. She is already taking some calls, and I'm sure starting to learn the limits of movement - how well you can text with one arm in a sling,' quipped then spokesman P.J. Crowley at the time, in an early reference to Clinton's communications habits.

Clinton was pictured wearing a sling emblazoned with the seal of the State Department when she returned to work. She also was photographed providing left-handed hand shake with a visiting Palestinian dignitary owing to her condition. She has showed no visible signs of lingering problems related to the injury.

HARD KNOCKS: Clinton got a fractured elbow in 2009, but still managed to negotiate with Hondouran leaders, and, according to her spokesman, text with one hand

2012 Blood Clot and Concussion

Clinton got a bad stomach bug and fainted at her home in Washington in 2012, an event that led her to get a concussion. Information about what exactly had happened emerged only slowly over time.

As her doctor put it, 'In December 2012, Mrs. Clinton suffered a stomach virus after traveling, became dehydrated, fainted and sustained a concussion.'

The then-secretary of state wasn't seen in public between Dec. 7th and when she left the hospital in New York January 2, 2013.

Clinton experienced 'double vision for a period of time and benefited from wearing glasses with a Fresnel Prism,' a special corrective lens, her doctor wrote in a letter voluntarily released to the media in 2015 as part of Clinton's presidential campaign. Her concussion 'resolved within two months,' Bardack wrote.

In 2014, Bill Clinton revealed that the injury 'required six months of very serious work to get over.' The former president called it a 'terrible concussion'

Clinton was diagnosed with a blood clot in the brain, transverse sinus venous thrombosis, and began anticoagulation therapy, her doctor wrote.

Clinton had to work from home and postpone planned testimony before a House Benghazi committee.

Clinton leaves New York Presbyterian Hospital with husband Bill and daughter Chelsea on January 2, 2013. The secretary of state, had not been seen in public since Dec. 7

Hypothyroidism

Clinton also suffers from Hypothyroidismrefers to an under-active thyroid gland, resulting in a lack of important hormones.

Clinton's doctor identified the condition in her 2015 letter, but did not state for how long Clinton has suffered from the condition. She takes a medication called Armour Thyroid.

Allergies

Clinton suffers from 'seasonal allergies,' according to her physician. It isn't known for how long she has suffered from allergies, although Clinton herself has cited her allergies when she has developed a cough – including on-stage during public events.

Her doctor states that Clinton is taking antihistamines, which treat the effects of allergies.

2016 Collapse and Pneumonia Diagnosis

Clinton had to leave a Sept. 11th service in New York early after spending 90 minutes at the ceremony. Her staff first cited heat and exhaustion, then ultimately revealed that Clinton had been diagnosed in pneumonia on Friday.

Clinton campaign spokesman Brian Fallon did not reveal what type of pneumonia Clinton has during a Monday interview on MSNBC, but said Clinton would be putting out more medical information.

'She was put on antibiotics and advised to rest and modify her schedule,' said Bardack in a statement released at the end of the day. While attending the event, 'she became overheated and dehydrated. I have just examined her and she is now re-hydrated and recovering nicely.'

Hours after Clinton was taken away from the Sept. 11th ceremony, her office released a doctor's statement. 'Secretary Clinton has been experiencing a cough related to allergies. On Friday, during follow-up evaluation of her prolonged cough, she was diagnosed with pneumonia. She was put on antibiotics, and advised to rest and modify her schedule. While at this morning's event, she became overheated and dehydrated. I have just examined her and she is now rehydrated and recovering nicely,' Clinton's doctor, Lisa Bardack, said in a written statement Sunday.'

'There's no other undisclosed condition. The pneumonia is the extent of it,' Clinton campaign spokesman Brian Fallon told MSNBC.

Fallon also acknowledged: 'I think in retrospect we could have handled it better in terms of providing more information more quickly.'

Campaign manager Robby Mook, speaking of the 90 minute delay before the press was told Clinton's status after she left the New York event, said Monday: 'We wish that that had been a lot shorter and that's on us.'

Clinton herself tweeted Monday: 'Thanks to everyone who’s reached out with well wishes! I’m feeling fine and getting better,' signing the missive with the letter 'H.'

Clinton had to shake with her left hand following her 2009 injury. She is pictured here with Palestinian Authority Salam Fayyad

Clinton campaigned in New York in June along with aide Huma Abedin. She was diagnosed with pneumonia Friday, according to her office

Clinton is 68 and would be 69 when she assumed office, second to Ronald Reagan in age taking office. Donald Trump is 70, and has released few details about his medical background.