Sepsis is the body’s extreme response to an infection. It is a life-threatening medical emergency. Sepsis happens when an infection you already have triggers a chain reaction throughout your body. Infections that lead to sepsis most often start in the lung, urinary tract, skin, or gastrointestinal tract. Most sepsis is caused by bacterial infections, but it can also be caused by viral infections, such as COVID-19 or influenza; fungal infections; or noninfectious insults, such as traumatic injury. Normally, the body releases chemical or protein immune mediators into the blood to combat the infection or insult.