King Charles III has been diagnosed with cancer and has started treatment in London.

The shock diagnosis comes a week after the 75-year-old British monarch left hospital following a separate procedure for an enlarged prostate.

Here’s what we know.

How was the cancer identified?

Charles’ recent health problems started last month when Buckingham Palace announced on January 17 that the King would attend hospital to undergo a “corrective procedure” for a benign enlarged prostate.

He had been diagnosed after experiencing symptoms and receiving a check-up while at his Birkhall residence in Aberdeenshire, Scotland.

He was discharged from the London Clinic on January 29 and was previously said to have been “doing well” after spending three nights at the private hospital near Regent’s Park. He was released hours after Catherine, Princess of Wales, left the same facility where she had been resting following a successful abdominal operation.

However, while the King was undergoing that treatment, a separate issue of concern was noted, according to the palace on Monday, and subsequent tests identified “a form of cancer.”

Health experts suggest it is not uncommon for many cancer patients to be diagnosed when seeking imaging or medical care for other reasons.

“You’re going in for one thing, but then you do additional testing — either as part of a general evaluation or influenced by certain symptoms or signs or blood tests that trigger a procedure or imaging — which then leads to the diagnosis of cancer in a different organ system,” said Dr. Anil Rustgi, director of the Herbert Irving Comprehensive Cancer Center at NewYork-Presbyterian/Columbia University Irving Medical Center, who was not involved in Charles’ medical care.

What don’t we know?

The palace has also not said how serious the cancer is or how early it was detected, nor did it specify what type of treatments the monarch might be receiving. A royal source told CNN that it was not prostate cancer but did not specify further.

The palace has also not said how serious the cancer is or how early it was detected, nor did it specify what type of treatment the monarch is undergoing.

Specific medical conditions of members of the royal family are rarely divulged publicly. The palace’s perspective is that they are entitled to some level of medical privacy despite their positions as public servants. That was the case with the initial diagnosis of an enlarged prostate. But Charles opted to share his diagnosis as he wanted to encourage other men who may be experiencing symptoms to get checked.

The situation is different when a condition could affect public duties. At that point, the palace has a duty to reveal what is going on, which is why a statement was issued on Monday evening.

How is King Charles doing?

King Charles was seen for the first time since leaving hospital on Sunday. With Queen Camilla by his side, he appeared to be in good spirits, waving to the public as the pair made their way to St. Mary Magdalene Church in Sandringham, Norfolk, for a Sunday morning church service.

He later returned to London and began outpatient treatment, before leaving the capital on Tuesday.

On the advice of his doctors, the King is postponing his public-facing engagements as he receives treatment but will continue with state business and official paperwork, the palace said.

He “remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible,” it added.

That means he’s likely to carry on getting his daily red boxes with government papers so he can continue working through state documents at home. CNN understands that it is anticipated the King will continue his weekly audience with the prime minister and that alternative arrangements will be made if his doctors advise him to minimize in-person contact.

It is also understood that he will continue to be available for state duties like Privy Council meetings. However, details of how that will occur are still being worked out.

The King traveled to Sandringham House, the royal family’s private country retreat in Norfolk, eastern England, on Tuesday after visits from his son, Prince Harry, and niece, Princess Beatrice.

Who fronts the family now?

Prince William returned to public duties on Wednesday for the first time since his wife, Catherine, had surgery last month. It is unclear what her surgery was for, but a royal source told CNN on January 17 that the 42-year-old’s condition was non-cancerous.

The announcement that William would be resuming some royal engagements came just hours before the King’s cancer diagnosis. William is in regular contact with his father, according to a source close to the Prince of Wales.

Prince William’s first public appearance was to host an investiture ceremony at Windsor Castle. He was expected to attend a charity gala for the London Air Ambulance Charity, where he’ll meet with crew members, supporters of the charity, and former patients of the charity on Wednesday evening.

The Prince of Wales had taken time off to support his family as his wife continued her recovery at their home in Windsor. It is expected that the princess will need to recuperate for several months, as Kensington Palace previously said she was unlikely to resume her public duties until after Easter. A royal source told CNN last week that the princess’ return to her official duties will depend on medical advice closer to the time.

Queen Camilla has been undertaking a full program of public duties in recent weeks, with that expected to continue. Meanwhile, other working members of the family are continuing their public engagements and CNN understands they could also pick up some additional duties on Charles’ behalf if needed.

The number of public-facing “working royals” has dwindled in recent years as “the Firm” sought to rebrand in step with the times. Only working royals carry out engagements on behalf of the King, splitting the 2,710 visits and events of last year between them.

That group was supposed to consist of 14 family members: King Charles, Queen Camilla, Princess Anne, Prince Andrew, the Duke and Duchess of Edinburgh, the Waleses, the Sussexes, the Duke and Duchess of Gloucester, and the Duke and Duchess of Kent. That is, until Prince Harry and Meghan chose to step down and Andrew was forced to, in the light of his relationship with the late pedophile Jeffrey Epstein. Now, 11 members of the clan conduct royal duties - more than half of whom are over the age of 70.

Harry returned to the United Kingdom on Tuesday to see his father following his diagnosis. The duke – who stepped back from royal duties in 2020 – made a transatlantic dash from California to see the King, who it is thought he hasn’t seen since the coronation in May.

What could happen if the King becomes too ill to work?

While all the signals coming from the palace are positive, there are constitutional provisions in place if the King is unable to temporarily carry out his official duties. In that instance, “counsellors of state” can be called upon to step in for him.

Two counselors can be appointed to act on the monarch’s behalf through what’s known as a letters patent and help keep the state ticking over. They would be authorized to sign documents, attend Privy Council meetings, and receive new ambassadors, but not perform some of the most important constitutional roles, such as appointing a Prime Minister. CNN understands there are no current plans to appoint any counselors.

The list of royals who can step in include Queen Camilla, Princes William, Harry and Andrew, and Princess Beatrice. In 2022, the King expanded this pool of family members to include his siblings, Princess Anne and Prince Edward. If this option was enacted over the coming weeks or months, it is unlikely the Dukes of Sussex or York would be directed to step up as they are no longer working royals.

If the King becomes completely unable to carry out his constitutional duties and the state can no longer function properly, his powers can be withdrawn and assumed by a regent. Under the Regency Act 1937, that would be the next in line to the throne, which is Prince William.

For that to happen, there has to be medical evidence “that the Sovereign is by reason of infirmity of mind or body incapable for the time being of performing the royal functions” or “is for some definite cause not available for the performance of those functions.”

A panel of four needs to be satisfied with the evidence by a majority vote. That panel consists of the Lord Chancellor, the Speaker of the House of Commons, the Lord Chief Justice of England, the Master of the Rolls, and The Queen. They would need to declare their decision in writing and would also declare if or when the King is ready to resume his duties. In the meantime, Prince William would act in his name.

What is the current line of succession?

The changes to the number of working royals in recent years has not changed the line of succession.

The King’s elder son, Prince William, is the first-in-line to the British throne. He is followed by his three children: 10-year-old Prince George, then Princess Charlotte, 8, and Prince Louis, 5.