Hillary Clinton embraced a young child after leaving her daughter's apartment - despite suffering from pneumonia.

The Democratic presidential candidate was diagnosed with the illness after she collapsed at an 'incredibly stifling' 9/11 ceremony in New York.

Pneumonia is a contagious illness that is spread from person-to-person when small droplets of water that contain the bacteria get into the air and people breathe them in.

Those infected usually spread the disease by sneezing while in close contact with others, who then breathe in the bacteria, according to the Centers for Disease Control and Prevention.

William Schaffner, an infectious disease expert at Vanderbilt University, said: 'Candidates are constantly out in enclosed spaces, face to face with myriads of people. It's an ideal opportunity for the transmission of a respiratory virus.'

Hillary Clinton was spotted bending down and speaking to the child after leaving her daughter's apartment yesterday

Clinton has cancelled a trip to California to attend fundraising events after it emerged the Democratic presidential nominee has pneumonia and been advised to rest by her doctor

Clinton left her daughter's apartment at about 11.45am, smiling and waving to a scrum of cameras and posing for a picture with a young girl before stepping into a campaign vehicle

Clinton said 'I'm feeling great. It's a beautiful day in New York', before heading for her home in Chappaqua, in New York state

Clinton was diagnosed with pneumonia on Friday and given antibiotics but had become dehydrated at the event in New York.

A dramatic video recorded by a bystander showed the Democratic candidate's knees buckling as she stepped off a kerb into a waiting van.

She had to be helped up by two Secret Service guards as her team rushed to help her and stopped her hitting the ground.

Clinton left after just 90 minutes of the ceremony at Ground Zero to mark the 15th anniversary of the September 11 attacks.

Her spokesman Nick Merrill said she felt 'overheated' and she was taken to her daughter Chelsea's apartment in the city in what was described as a 'medical episode'.

Another picture taken moments earlier showed the 68-year-old holding her chest with her right hand and looking uncomfortable.

Representative Joe Crowley, a New York Democrat, described the memorial as being 'incredibly stifling'.

Video surfaced of Clinton appearing to stumble as she was led into a van after suffering a 'medical episode' during the 9/11 memorial service

Her aides then rushed to her side and then helped her into the van before she was rushed off to Chelsea's Manhattan home

An hour after arriving at her daughter's apartment, she emerged looking relaxed.

Asked if she was feeling better, Mrs Clinton said: 'Yes thank you, very much'.

She smiled and posed for pictures with the young girl before leaving for her home in Chappaqua, just northeast of Manhattan.

'I'm feeling great, it's a beautiful day in New York,' Clinton said.

Following her collapse she was forced to cancel a California fundraising trip on Monday and Tuesday.

Her campaign team also faced questions as to why it took them two days to reveal the illness.

Clinton emerged from her daughter Chelsea's apartment after she was rushed from the 9/11 memorial service because of a 'medical episode'

The diagnosis puts the health of Clinton back into the spotlight with two months left in the US presidential election.

She has suffered numerous coughing fits on live TV, joking how 'every time I think of Trump I get allergic'.

Her critics have seized on photos of her which they claim show she has suffered seizures.

Should Mrs Clinton win the presidency she will be 69 when she takes office, making her the second oldest person to assume the title after Ronald Reagan.