Sepsis Causes

While sepsis can stem from infections caused by bacteria, fungi, or viruses, it is not the infection itself that defines the condition, but the body’s overwhelming and often harmful immune response.

Bacterial infections are the most frequent cause of sepsis and can arise from various parts of the body, depending on the source and severity of the infection. While most bacterial infections are managed effectively by the immune system or through medical intervention, some can overwhelm the body’s defences and lead to sepsis.

One of the most common sources of sepsis is bacterial pneumonia, an infection that affects the lungs. Bacteria such as Streptococcus pneumoniae or Haemophilus influenzae are typical culprits. When pneumonia is not treated promptly or effectively, bacteria can spread from the lungs into the bloodstream, causing systemic inflammation. This is particularly dangerous in individuals with pre-existing respiratory conditions, such as chronic obstructive pulmonary disease (COPD), or in those with weakened immune systems, such as elderly patients. Symptoms of sepsis from bacterial pneumonia often include difficulty breathing, fever, and confusion, which can escalate quickly if not addressed.

Urinary tract infections (UTIs) caused by bacteria are another significant source of sepsis, especially in older adults, individuals with diabetes, or those with indwelling catheters. Common bacteria such as Escherichia coli are often responsible. While UTIs typically start with localised symptoms such as pain during urination, frequent urination, or a burning sensation, if left untreated, the bacteria can ascend to the kidneys, leading to a more severe infection.

Bacterial infections in the abdomen, such as appendicitis or peritonitis, can also lead to sepsis. These infections occur when bacteria like Escherichia coli or Bacteroides fragilis, which are naturally present in the gut, breach the intestinal walls due to inflammation, injury, or surgery. This allows bacteria to spread into the abdominal cavity, triggering severe infection and inflammation. If untreated, the bacteria can enter the bloodstream, causing widespread septic complications. Abdominal sepsis is particularly dangerous because the early symptoms, such as stomach pain or nausea, may be mistaken for less severe conditions, delaying treatment.

Skin and soft tissue infections are another route through which bacteria can cause sepsis. These infections often arise from wounds, cuts, or surgical incisions that become contaminated with bacteria. In some cases, these bacteria can cause severe conditions like cellulitis, abscesses, or even necrotizing fasciitis (commonly referred to as flesh-eating disease). These infections can spread quickly through the bloodstream, leading to sepsis if not identified and treated promptly. People with diabetes or compromised circulation are particularly at risk, as their wounds may heal more slowly, providing bacteria with an opportunity to proliferate.

Sepsis can also develop from bloodstream infections (bacteraemia) caused by bacteria entering directly into the blood through intravenous lines, surgical procedures, or other invasive medical devices. This is a particular concern in healthcare settings, where hospital-acquired infections caused by bacteria such as Methicillin-resistant Staphylococcus aureus (MRSA) or Pseudomonas aeruginosa can lead to severe complications. In these scenarios, lapses in sterilisation or infection control can introduce bacteria into the bloodstream, causing life-threatening sepsis.

While bacteria are naturally present in many parts of the body, it is the disruption of normal barriers—such as the skin, the mucosal lining of the gut, or the urinary tract—that allows harmful bacteria to invade sterile areas, triggering a systemic immune response.

Fungal infections, though less common than bacterial causes, can also lead to sepsis, particularly in individuals with weakened immune systems. Fungal infections like candidiasis, caused by the yeast Candida, can spread from localised sites, such as the mouth or genital areas, to the bloodstream, resulting in invasive candidiasis. These types of infections are more likely to occur in hospital settings or in individuals undergoing treatments such as chemotherapy or organ transplants, which suppress immune function. While minor infections may not pose a significant risk to healthy individuals, people with compromised immune systems, such as cancer patients undergoing chemotherapy, organ transplant recipients on immunosuppressive drugs, or individuals with advanced HIV are at heightened risk. In these individuals, the fungi can breach natural defences and enter the bloodstream, leading to fungemia, the precursor to sepsis. Once fungi enter the bloodstream, they can disseminate throughout the body, reaching organs such as the liver, kidneys, heart, and brain. This widespread fungal invasion triggers the immune system to release inflammatory mediators in an attempt to fight the infection. However, this immune response can spiral out of control, causing damage to healthy tissues and organs. The resulting systemic inflammation can lead to reduced blood flow, blood clotting abnormalities, and multi-organ failure, which are all hallmarks of sepsis.

Viral infections, including influenza, COVID-19, and others, can also trigger sepsis. Viruses may weaken the body’s defences and make it susceptible to secondary bacterial infections, or they may directly provoke an inflammatory response severe enough to cause sepsis. Influenza, a common respiratory virus, is a notable example. While most individuals recover within a week or so, the virus can cause severe complications in those with weakened immune systems, such as the elderly, pregnant women, or individuals with chronic illnesses. The infection can spread from the respiratory tract to other parts of the body, causing secondary bacterial infections or directly inducing an inflammatory response severe enough to result in sepsis.

The emergence of SARS-CoV-2, the virus responsible for COVID-19, has also brought significant attention to viral sepsis. Severe cases of COVID-19 are frequently marked by symptoms of sepsis, including acute respiratory distress, low blood pressure, and multi-organ dysfunction. In many cases, the virus not only causes direct damage to lung tissue but also initiates an overactive immune response. This combination of viral replication and immune dysregulation can lead to a sepsis-like state, particularly in patients with underlying health conditions.

Sepsis is not a condition that can be “caught” from another person. It is a complication that arises uniquely in individuals based on their immune system’s response to an infection. While the bacteria, fungi, or viruses that cause infections can sometimes spread between individuals, sepsis itself is an internal reaction that depends on the host’s health, the type of infection, and how quickly it is treated.