Blood poisoning

Sepsis can be triggered by an infection in any part of the body.

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 occurs, such as tissue swelling, which helps fight the infection and prevent its spread. This process is known as inflammation.

If your immune system is weakened or if an infection is particularly severe, the infection can 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, leading to a dangerous drop in blood pressure, which stops oxygen reaching your organs and tissue.

Sources of infection

The most common sites of infection leading to sepsis are the lungs, urinary tract, abdomen and pelvis. Types of infection associated with sepsis include:

• lung infection (pneumonia)
• flu (influenza)
• appendicitis
• infection of the lining of the digestive system (peritonitis)
• an infection of the bladder, urethra or kidneys (urinary tract infection)
• skin infections, often caused when an intravenous drip or catheter has been inserted into the body through the skin
• post-surgical (after surgery) infections
• infections of the nervous system, such as meningitis or encephalitis

In approximately one in five cases, the infection and source of sepsis cannot be detected.

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, such as HIV or leukaemia, that weakens their immune system
• people receiving medical treatment, such as chemotherapy, that weakens their immune system
• the very young or very old
• people who have just had surgery or who have wounds or injuries as a result of an accident
• people on mechanical ventilation
• people with drips or catheters attached to their skin
• people who are genetically prone to infection

Sepsis is a particular risk for people who are already in hospital due to another serious illness. Despite the best efforts of doctors and nurses, secondary infections acquired in hospital are always a potential risk.

Hospital-acquired bacterial infections, such as MRSA, tend to be more serious as the bacteria causing the infection have often developed a resistance to antibiotics.