Queen Elizabeth's health has been the subject of worry and speculation since October last year, when she stayed overnight in hospital for the first time since 2013.

Now, just as she was resuming her regular duties, the 95-year-old has been diagnosed with COVID-19.

Buckingham Palace has always been reluctant to discuss the Queen's health, but here is what's been made public over the last few months.

Two cancellations and a hospital stay

The Queen cancelled a two-day trip to Northern Ireland on medical advice in October 2021, and instead rested for several days.

Just before that, she'd had a hectic few days in which she held audiences with diplomats, had a reception at Windsor Castle for global business leaders, and attended the races at Ascot.

The advice from her doctors to rest came days after she was seen using a walking stick for the first time in 20 years. 

Pictures of her using the walking stick circulated at the same time Vanity Fair reported doctors had advised her to forego the consumption of alcohol, citing her advancing years.

Later the same month, she stayed overnight in hospital, for what Buckingham Palace called "primary investigations", but it was not related to COVID.

A royal source cited by Reuters said the Queen had stayed in hospital for practical reasons and that her medical team had taken a cautious approach.

Near the end of October, she was forced to cancel her long-standing plan to attend the COP26 Climate Conference in Glasgow, opting to send a video message instead.

Her doctors then advised her to rest for at least two weeks and undertake only light desk duties.

Remembrance services missed

The Queen, the head of the British armed forces, was then forced to miss annual remembrance services in November.

The palace said the Queen would not attend a festival of remembrance scheduled for November 13, but had a "firm intention" to be present at a national service of remembrance for military veterans on November 14.

Unfortunately, that too fell through when she sprained her back and Buckingham Palace had to announce she wouldn't be attending just two hours before the service.

Omicron forces Christmas celebrations at Windsor

Aside from her health, the Queen also kept a low profile due to the spread of the Omicron variant of COVID-19 in the United Kingdom.

For Christmas, instead of her usual choice of celebrating at her Sandringham estate in Norfolk, the Queen decided to spend the holiday at Windsor, a palace source said.

Omicron infections were sweeping across the country at the time.

The previous week, she cancelled a pre-Christmas lunch with her extended family as a safety precaution against the latest variant.

Platinum Jubilee without Charles

The Queen's COVID-19 infection comes just weeks after she celebrated 70 years on the throne.

To mark the occasion, the Queen attended an in-person Platinum Jubilee celebration on February 6.

Her eldest son, Prince Charles, however, pulled out of the event after receiving news of a positive COVID test.

A palace source said he had met with the Queen days earlier.

In mid-February, she made her first public appearance since Prince Charles' COVID diagnosis.

The Queen held virtual audiences via video call, and received both the outgoing and incoming Defence Services Secretaries at Windsor Castle.

The Queen quipped to members of the royal household that she could not move much as she carried out her duties, and a palace source said the Queen had been feeling slightly stiff, rather than having injured herself or being unwell.

She had been about to resume her normal duties following her Platinum Jubilee when she contracted COVID-19.

Wires/ABC