LONDON (Reuters) - Pope Francis, the 88-year-old head of the 1.4-billion-member Roman Catholic Church, is in hospital being treated for double pneumonia following a respiratory infection.

WHAT IS DOUBLE PNEUMONIA?

Pneumonia is inflammation of the lungs, usually caused by a viral or bacterial infection. The pope has bilateral pneumonia, known informally as double pneumonia, and means both lungs are affected.

When a person has pneumonia, small sacs in the lungs called alveoli are filled with pus and fluid, which makes breathing painful and restricts oxygen intake.

Symptoms can include coughing, including coughing up mucus, shortness of breath, a temperature, aches and pain, exhaustion, and confusion.

HOW SERIOUS IS IT?

Many people recover from pneumonia in a few weeks, but a number of factors can make it more dangerous. It is often more risky for babies and adults over 65, as well as those with cardiovascular disease or pre-existing lung conditions.

The pope is especially prone to lung infections because as a young adult he had pleurisy and part of one lung removed.

He developed pneumonia after being hospitalised last week for a respiratory infection on top of two other airway conditions, bronchiectasis and asthmatic bronchitis, a Vatican statement said on Tuesday.

Bilateral pneumonia is more serious because it affects both lungs, meaning the patient has less healthy lung capacity remaining to breathe with, doctors said.

HOW COMMON IS PNEUMONIA?

Pneumonia is common. Alongside other lower respiratory tract infections, the World Health Organization says it is one of the leading causes of death worldwide.

For example, in the United States, around one million older adults are hospitalized with pneumonia annually, and around 50,000 die, according to the American Thoracic Society.

Globally, it kills 700,000 children under the age of 5 annually, the WHO estimates.

HOW IS IT TREATED?

Pneumonia can be treated with antibiotics to tackle the bacterial cause of the infection or anti-viral drugs can help fight a viral infection.

In hospital, people often get fluids and oxygen too.

The Vatican described the pope's initial respiratory infection as "polymicrobial", which means it is caused by a combination of pathogens. That made it more complex to treat, and he was receiving antibiotics and corticosteroids.

He is now also getting "additional pharmacological treatment" for the pneumonia, the Vatican said.

(Reporting by Jennifer Rigby; Editing by Crispian Balmer and Janet Lawrence)