Lily Allen's hospitalisation with septicaemia has focused attention on a little understood but potentially fatal condition which affects thousands of people every year.

The 25-year-old singer was responding to treatment after being taken to hospital by ambulance from her Gloucestershire home on Friday night, where she was recovering from her second miscarriage. She had been with her boyfriend, Sam Cooper, 32, after losing her baby five days earlier, six months into her pregnancy.

Septicaemia is a recognised risk following miscarriage, as foetal material can be left behind in the womb, acting as a reservoir for infection.

The condition can be life-threatening and is caused by the body over-reacting to an infection. The immune system goes into overdrive, triggering reactions that can lead to widespread swelling and blood clotting.

Septicaemia is often called blood poisoning, although it can affect the whole body, including the organs.

When the function of vital organs such as the heart, lungs, kidneys or liver is affected, urgent treatment with intravenous antibiotics in intensive care is necessary.

There are more than 30,000 cases of severe sepsis in the UK every year and the numbers appear to be rising.

Septicaemia can be caused by an infection in any part of the body, including the lungs (flu or pneumonia), urinary tract, skin or nervous system (meningitis).

Normally, the body responds by fighting the infection, sending white blood cells to destroy the germs causing it. But if the immune system is weakened or the infection is severe it can spread.

The immune system then overreacts, causing inflammation which damages tissues, interferes with the flow of blood and can lead to death.

Symptoms of septicaemia develop quickly and include fever, chills, a rapid heartbeat and breathing, followed by dizziness (indicating low blood pressure), nausea and cold, clammy skin.

Allen is likely to have been treated in intensive care with intravenous antibiotics designed to work against a wide range of bacteria, while the results of laboratory tests are awaited.

Once a specific bacterium is identified, doctors can give her a more focused antibiotic.