Hillary Clinton has pneumonia: Here are five things you should know about the disease

US presidential candidate Hillary Clinton has cancelled campaign engagements after being diagnosed with pneumonia.

The 68-year-old fell ill at a ceremony to mark the 15th anniversary of the September 11 attacks, and appeared to lose her footing as she was helped into her vehicle.

Here are five things to know about the disease.

What is pneumonia?

Pneumonia is an acute respiratory infection caused by bacteria, viruses or fungi that target the lungs.

When it strikes, the tiny air sacs inside the lungs fill up with pus and fluid, limiting oxygen intake and making it painful for the patient to breathe.

All types of pneumonia share similar symptoms including cough and fever, with wheezing being more common in viral cases.

Doctors usually diagnose pneumonia by conducting a physical exam, checking symptoms and carrying out a chest X-ray to ensure that the patient is not suffering from bronchitis or other respiratory infections.

Fungal pneumonia tends to be relatively rare, affecting people with weakened immune systems, such as AIDS patients, who become sick either when they inhale wind-borne spores or when a fungus already present in their bodies triggers symptoms.

Who is at risk?

Pneumonia can affect people of all ages but it can be particularly serious, even life-threatening, for the very old or the very young.

Smokers, as well as people with underlying medical conditions such as asthma, diabetes and heart disease, are particularly vulnerable, according to the US Centers for Disease and Control Prevention (CDC).

The infection is spread via air-borne droplets from coughs or sneezes.

Doctors say hand-washing is an effective way to reduce the risk of picking up pneumonia.

How dangerous is it?

When an otherwise healthy adult under 65 years of age gets pneumonia, his or her body can usually fight off the infection.

But patients with a weak immune system -- those who are very young or old, or those who have a pre-existing condition -- can struggle to cope and will sometimes develop complications.

As the infection sets in, it can cause oxygen levels in the body to fall. In extreme cases, this can lead to organ failure and death.

The World Health Organization calls pneumonia the single biggest killer of children under five worldwide, responsible for 15 percent of deaths in this age group every year.

What is the recommended treatment?

It depends on what kind of pneumonia it is.

Doctors advise patients to take rest and consume plenty of fluids.

The normal treatment includes antibiotics for bacterial cases, while viral pneumonia is treated with antiviral drugs and the occasional prescription of corticosteroid medicines.

Fungal pneumonia is usually treated with antibiotics or antifungal medication.

Hospitalisation is recommended in high-risk cases.

What is the usual recovery time?

Recovery time depends on the severity of the symptoms before treatment but in most cases, patients feel better after a course of medicines and a week of rest.