What is acute myeloid leukaemia, as seen in Call the Midwife?

But (spoiler alert for episode two!) Fiona hadn't been home long before she collapsed, and was readmitted to the Lady Emily. She was soon diagnosed with acute myeloid leukaemia.

What is acute myeloid leukaemia?

"Leukaemia" is cancer of the white blood cells, and "acute leukaemia" means that the cancer progresses quickly and aggressively.

As for the "myeloid" bit: there are two main types of white blood cells – lymphocytes and myeloid cells. "Acute myeloid leukaemia" is a cancer of the myeloid cells, which are blood cells that do all sorts of different things: fighting bacterial infections, defending the body against parasites, and preventing the spread of tissue damage.

This rare cancer is mainly diagnosed in older people, but adults and children can also get it – as in the case of Call the Midwife's Fiona Aylward (Jo Herbert), who is perhaps in her 30s when she is diagnosed. According to the NHS, only around 3,100 people are diagnosed with acute myeloid leukaemia in the UK each year.

What are the symptoms of acute myeloid leukaemia?

AML can be difficult to identify, because the symptoms are quite non-specific – and they cross over with a lot of other illnesses.

Symptoms can include general weakness, fatigue, fever, susceptibility to other infections and illnesses, easy bruising and bleeding, weight loss, breathlessness, pain in the bones or joints, a feeling of fullness in the tummy area, pale skin and (less commonly) swollen lymph nodes.

In Call the Midwife, Fiona Aylward is originally diagnosed with an iron deficiency during her pregnancy. However, when she is hospitalised again with more troubling symptoms, Mr Scarisbrick (Richard Dillane) takes another look at her old blood test results and finds them "not entirely consistent with a straightforward iron deficiency."

Tests later reveal she has acute myeloid leukaemia. But Mr Scarisbrick didn't miss anything obvious, because the illness can be very hard to spot when it begins. And the condition has nothing to do with her pregnancy or childbirth, but just happened to develop at the same time.

In Call the Midwife, Fiona Aylward also develops a rash in the crook of her arm – which understandably alarms Trixie, as she lost her friend Nurse Barbara (Charlotte Ritchie) to septicaemia.

However, according to cancer.org, "If leukaemia cells spread to the skin, they can cause lumps or spots that may look like common rashes." And healthline.com explains: "The red spots are caused by tiny broken blood vessels, called capillaries, under the skin. Normally, platelets, the disc-shaped cells in the blood, help the blood clot. But in people with leukaemia, the body doesn’t have enough platelets to seal off the broken blood vessels."

How is AML diagnosed?

In Call the Midwife, the Lady Emily Clinic initially orders blood tests for Fiona. But as her mysterious illness becomes worse, Mr Scarisbrick calls in a haematologist from St Luke's Hospital, who performs a "bone marrow aspiration" – that is, he takes a sample of soft bone tissue under local anaesthetic.

He analyses that sample, and gives his diagnosis.

In the present day, the process is still quite similar. A GP will order blood tests to check for a high number of abnormal white blood cells, or a very low blood count. If a problem is found, the patient is referred to a haematologist, who carries out further tests.

To confirm a diagnosis, a bone marrow biopsy is performed. Nowadays, the doctor will take the sample of liquid bone marrow from the back of the hip bone, and it will be checked for cancerous cells.

Doctors might then perform more tests to establish the progress and extent of the disease. Genetic testings on the bone marrow and blood samples may also be carried out to discover more about the specific type of AML.

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Can AML be treated?

For Fiona Aylward, there is no real hope of recovery. Mr Scarisbrick can only offer a blood transfusion to try to make her feel a bit better.

But in the present day, there are more treatments available. Because the condition usually comes on suddenly and progresses quickly, treatment must be started as soon as possible after diagnosis.

Chemotherapy is the main treatment used – ideally an intensive, high-dose regimen, if the patient's body is considered able to withstand it. Further treatment may also involve a bone marrow or stem cell transplant.

Generally, patients are also given regular blood transfusions, and closely monitored for infections.

What is the prognosis? Is AML fatal?

According to Cancer Research UK, "Around 20 out of 100 people (around 20 per cent) will survive their leukaemia for five years or more after their diagnosis."

That's based on the NCIN's figures for England (2008-10), and – as the charity notes – some people with AML survive much longer than five years, and go into remission.

Younger people have a much better chance of surviving the disease, but there a lot of factors that feed into a patient's prognosis – including how far the leukaemia has progressed, how well it responds to treatment, and specific genetic abnormalities in the leukaemia cells.

Call the Midwife continues on Sundays at 8pm on BBC One. While you're waiting, take a look at the rest of our Drama coverage, or check out our TV guide.