Vicious killers on Halloween tend to be knife-wielding murderers, hoodlums in masks or poison apples. But the Daily Mail has published a story about a teen in England who nearly died from sepsis, a reminder that some of the scariest assailants are the tiny organisms inside us.

The 18-year-old apparently thought she had tonsillitis or the flu, but when she became even sicker and her mother rushed her to the hospital, doctors found she had the dangerous blood infection. “It had spread throughout her body, causing serious damage to her lungs, uterus and ovaries as doctors warned her family she may not survive,” the Daily Mail said. “Surgeons removed her left ovary and placed a tube in her neck to help her breathe in a desperate attempt to keep her alive. She spent four weeks in an induced coma in intensive care, and when she eventually woke up she had no recollection of what had happened.”

Now 19 and back at work, the woman is still not completely back to normal, despite months of physiotherapy: “She still gets breathless and tired. It has also caused her hair to thin.”

According to the Centers for Disease Control and Prevention, sepsis is a complication of infection, stemming from “the body’s overwhelming and life-threatening response to an infection which can lead to tissue damage, organ failure and death.” The Mayo Clinic explains that it specifically occurs when the body releases chemicals into the bloodstream to fight off the intruding infection and those chemicals “trigger inflammatory responses throughout the body. This inflammation can trigger a cascade of changes that can damage multiple organ systems, causing them to fail.” The worse sepsis gets, the more blood flow is cut off from vital organs, and it can cause blood clots in those organs and in your extremities that can lead to gangrene.

Some of the most common types of infections that lead to sepsis, the CDC notes, are lung infections like pneumonia and infections in the urinary tract, skin and gut; germs like strep and E.coli can also cause the condition. It sometimes requires surgery to eradicate but sepsis can be treated with antibiotics and hospital care such as oxygen and intravenous fluids.

“Know that time matters,” the CDC says. “If you have a severe infection, look for signs and symptoms like: shivering, fever, or very cold, extreme pain or discomfort, clammy or sweaty skin, confusion or disorientation, short of breath, and high heart rate.”

About 1 million people in the U.S. suffer severe sepsis every year, according to the National Institutes of Health. Of that group, between 28 and 50 percent die, “far more than the number of U.S. deaths from prostate cancer, breast cancer and AIDS combined.”

Some famous deaths from sepsis include actress Patty Duke, who famously played Helen Keller and starred in the Patty Duke Show; Pope John Paul II; Grease actor Jeff Conaway; President James A. Garfield; boxer Muhammad Ali; and Superman actor Christopher Reeve.