The Duke of Edinburgh has been admitted to hospital with a bladder infection for the second time in just over two months.
Elderly people tend to be more prone than most to infections of the urinary tract, including the bladder. They are found in about one in 10 men in their 80s and can recur.
How common are the infections?
In general, the elderly are more vulnerable to infections because the immune system weakens with age.
The muscles of the bladder also get weaker which can make it harder to completely empty it when urinating.
This means bacteria that can cause infection may get more of a chance to grow in the residual urine left behind.
How else can they be caused?
In some elderly men an enlarged prostate gland may lead to obstructed urinary flow. Kidney stones may also cause problems.
What are the symptoms?
Symptoms of a urinary tract infection include pain when urinating, cloudy, bloody or smelly urine, and a need to go to the toilet very often.
How serious are these infections?
It can be more difficult to spot infection in elderly people as some don't develop a fever - as younger sufferers often do - because their immune systems are not powerful enough to swing into action to fight the problem.
Sometimes these infections - if not picked up an early stage - may also cause confused thought.
However, the vast majority of sufferers improve within a few days of starting antibiotic treatment.
Why might the Duke of Edinburgh be especially vulnerable?
Prince Philip has long been recognised as a very fit and active man for his age - 91.
But in December last year he underwent treatment for a blocked coronary artery.
He was fitted with a metal tube known as a coronary stent, designed to keep the blocked artery open. It also reduces the pain from the blockage, caused by a lack of blood supply to the heart tissues.
In some instances patients are given drugs to minimise the risk of complications, such as over-growth of new tissue, after their stent is fitted. These drugs may suppress the immune system.
If the duke received this form of treatment it is possible that made him less able to fight off infections, such as the bladder infection in June, which now appears to have recurred.