Sepsis can be triggered by an infection in any part of the body. The most common sites of infection leading to sepsis are the lungs, urinary tract, abdomen (tummy) and pelvis.

Sepsis may develop when you are already in hospital, for example if you have recently had surgery, have a urinary catheter fitted or have to stay in hospital for a long time – all increasing your risk of developing sepsis.

Sources of infection

Types of infection associated with sepsis include:

• lung infection (pneumonia)
• appendicitis
• an infection of the thin layer of tissue that lines the inside of the abdomen (peritonitis)
• an infection of the bladder, urethra or kidneys (urinary tract infection)
• an infection of the gallbladder (cholecystitis) or bile ducts (cholangitis)
• skin infections, such as cellulitis (this can be caused by an intravenous catheter that has been inserted through the skin to give fluids or medication)
• post-surgical (after surgery) infections
• infections of the brain and nervous system, such as meningitis or encephalitis
• flu (in some cases)

Sometimes, the specific infection and source of sepsis cannot be identified.

What causes the symptoms of sepsis?

Usually, your immune system will keep the infection limited to one place (known as a localised infection). Your body will produce white blood cells, which travel to the site of the infection to destroy the germs causing infection. A series of biological processes occur, such as tissue swelling, which helps fight the infection and prevents it spreading. This process is known as inflammation.

If your immune system is weak or an infection is particularly severe, it can quickly spread through the blood into other parts of the body. This causes the immune system to go into overdrive, and the process of inflammation affects the entire body.

This can cause more problems than the initial infection, as widespread inflammation damages tissue and interferes with the flow of blood. The interruption in blood flow leads to a dangerous drop in blood pressure, which stops oxygen reaching your organs and tissues.

People at risk

Everybody is potentially at risk of developing sepsis from minor infections, such as flu. However, some people are more vulnerable, including people who:

• have a medical condition that weakens their immune system, such as HIV or leukaemia
• are receiving medical treatment, such as chemotherapy or long-term steroids, that weakens their immune system
• are very young or very old
• are pregnant
• have a long-term health condition, such as diabetes
• have just had surgery, or have wounds or injuries as a result of an accident
• are on mechanical ventilation (where a machine is used to help you breathe)
• have drips or catheters attached to their skin
• are genetically prone to infections

Sepsis is a particular risk for people already in hospital due to another serious illness. Despite the best efforts of doctors and nurses to avoid infections being acquired in hospital, people who fall into the categories described above are still at an increased risk of developing sepsis.

Hospital-acquired bacterial infections, such as MRSA, tend to be more serious as these bacteria have often developed a resistance to many commonly used antibiotics.