Chronic lymphocytic leukaemia is a type of cancer that affects the white blood cells and tends to progress slowly over many years.
It mostly affects people over the age of 60 and is rare in people under 40. Children are almost never affected.
In chronic lymphocytic leukaemia (CLL), the spongy material found inside some bones (bone marrow) produces too many white blood cells called lymphocytes, which are not fully developed and do not work properly.
Over time this can cause a range of problems, such as an increased risk of picking up infections, persistent tiredness, swollen glands in the neck, armpits or groin, and unusual bleeding or bruising.
CLL is different from other types of leukaemia, including chronic myeloid leukaemia, acute lymphoblastic leukaemia and acute myeloid leukaemia.
CLL does not usually cause any symptoms early on and may only be picked up during a blood test carried out for another reason.
When symptoms develop, they may include:
You should visit your GP if you have any persistent or worrying symptoms. These symptoms can have causes other than cancer, but it's a good idea to get them checked out.
Read more about diagnosing CLL and complications of CLL.
As CLL progresses slowly and often has no symptoms at first, you may not need to be treated immediately.
If it's caught early on, you'll have regular check-ups over the following months or years to see if it's getting any worse.
If CLL starts to cause symptoms or is not diagnosed until later on, the main treatments are:
A new type of treatment involves a stem cell or bone marrow transplant, where donated cells called stem cells are transplanted into your body so you start to produce healthy white blood cells.
This is an intensive type of treatment that's not suitable for everyone.
Treatment cannot usually cure CLL completely, but can slow its progression and lead to periods where there are no symptoms.
Treatment may be repeated if the condition comes back.
Find out more about treating CLL
The outlook for CLL depends on how advanced it is when it's diagnosed, how old you are when diagnosed, and your general health.
Generally, about 7 out of 10 people will survive their leukaemia for 5 years or more after being diagnosed.
Younger, healthier people who are diagnosed when CLL is still in the early stages generally have the best outlook.
Although it cannot normally be cured, treatment can help control the condition for many years.
It's not clear what causes CLL. There's no proven link with radiation or chemical exposure, diet or infections.
You cannot catch it from anyone else or pass it on.
But having certain genes can increase your chances of developing CLL. You may be at a slightly higher risk of it if you have a close family member with it, although this risk is still small.
Living with a serious and long-term condition such as CLL can be very difficult.
You may find it useful to find out as much as you can about the condition and speak to others affected by it.
The following support groups and charities can offer help and advice for people with CLL, their families and their carers:
The Macmillan Cancer Support and Cancer Research UK websites are also good places for CLL information and support.