Pneumonia, the disease that Hillary Clinton is suffering from in the middle of her election campaign, can become serious with a matter of hours and symptoms may last for months.

The disease, caused by a bacterial infection and defined by the swelling of the tissue in the lungs that it causes, can have lasting and damaging effects if it is not quickly treated.

Spotting pneumonia can often be difficult, because its symptoms might appear initially to be something else less severe like colds or asthma. It can include a cough, which might include mucus or be dry, and people might suffer difficulty breathing or shortness of breath.

Clinton faints as she leaves 9-11 memorial

Sufferers might also experience a rapid heartbeat, fever, sweating, shivering and chest pain. In more extreme circumstances, it might cause symptoms including coughing up blood, headaches, extreme tiredness, nausea and vomiting.

It is that tiredness that is thought to have hit Mrs Clinton when she left a 9/11 commemoration event early and appeared to be struggling to get into the car that was due to take her away. It appears that the effects of pneumonia –combined with various other things, including the intense heat of the day, led her to feel faint and have to leave the event early.

That fatigue can last for several months after the problem is first experienced.

The problem is not an uncommon one. In the UK, it affects about eight in every 1,000 adults and is especially common during winter months and those who are very young or older.

In pictures: Hillary Clinton's 2016 presidential bid Show all 30 1 /30 In pictures: Hillary Clinton's 2016 presidential bid In pictures: Hillary Clinton's 2016 presidential bid Clinton campaign 11/09/2016: Hillary Clinton waves to the press as she leaves her daughter's apartment building after resting in New York. 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It can also be more serious in people who are older. It might also be more dangerous to those that have existing health issues, like asthma, cystic fibrosis, heart, kidney or liver disease.

Mild presentations of the disease are usually treated by having rest, antibiotics and plenty of fluids. Sometimes more severe cases will need to be treated in hospital.

Pneumonia may become even more serious such as pleurisy, where the linings between the lungs and ribcage become inflames. It might also lead to others like a lung access and blood poisoning.

Mrs Clinton’s campaign released a letter last year that attested to her good health, in response to repeated questioning of whether she was well enough to lead.

That letter, from Dr Bardack, pointed to deep vein thrombosis that she suffered in 1998 and 2009, and a concussion in 2012. The letter said that she had completely recovered from those events, after some absence while she was treated.