If asked to ponder the most expensive health conditions in the U.S., most of us would list things like cancer, heart disease, and stroke — well known afflictions that have been consistently deadly for many years. According to the newest federal data, though, it’s a far more unfamiliar condition that takes the title of most expensive cause of hospitalization in the U.S.

Sepsis, a serious complication of infection, is much more common and deadly than many people seem to realize. About one million people are hospitalized for sepsis each year, which is more than heart disease and stroke combined. On top of this, sepsis is particularly dangerous for the elderly, and patients admitted into the hospital with sepsis have a high chance of dying from it.

In 2013 alone, $5.5 billion was spent on 400,000 Medicare beneficiaries who were hospitalized with sepsis. Once patients are in the hospital with sepsis, somewhere between one in eight and one in four will die from it, and 33 to 50 percent of all hospital deaths can be attributed, at least in part, to sepsis.

Sepsis undoubtedly has some grave consequences, but less than half of Americans even know what sepsis is.

What is sepsis?

Sepsis is the body’s life-threatening response to an infection. When the body is infected, it releases chemicals into the blood to fight the harmful pathogens. Normally, this would allow the body to fight off the infection, but sometimes the very chemicals meant to keep the body safe trigger an adverse inflammatory response. This inflammation can lead to many severe consequences, including tissue damage, multiple organ failure, and blood clots.

From a medical standpoint, defining exactly what sepsis is has been a problem for some time. The condition was last defined in 2001, and many question whether it is specific and effective enough to distinguish sepsis from other diseases with similar symptoms. Confusion has leaked out into public opinion as well — sepsis is often referred to as “blood poisoning,” an inaccurate term. Sepsis is the body’s immune response to an infection present anywhere in the body (most commonly pneumonia, a urinary-tract infection or an intra-abdominal infection), while blood poisoning refers to an infection present only in the blood.

Doctors have identified three stages of the disease: sepsis, severe sepsis, and septic shock. Sepsis can begin in response to something as simple as a playground scrape, or can occur while one is in the hospital recovering from a procedure. The condition escalates to septic shock when tiny blood clots are formed because of the inflammation, blocking oxygen flow to vital organs and causing a life-threatening drop in blood pressure.

How is it treated?

Once a patient has had their symptoms identified as sepsis, a doctor can treat the condition with a number of medications. Antibiotics to fight the infection are administered via IV, followed by corticosteroids to reduce inflammation. Vasoactive medications to increase blood pressure and insulin for sugar stability can also play a part in treatment. In severe cases, surgery may be necessary to remove the source of the infection. Painkillers, of course, are often administered as well.

Though sepsis is clearly a complex condition, as little as ten years ago doctors figured if a patient could survive long enough for a hospital discharge, they were no longer in critical danger. This simply isn’t true—about 40 percent of sepsis patients go back to the hospital within three months of getting discharged. Improvements in care have lagged behind those concerning heart disease and cancer — attention has shifted to chronic diseases lately. This irregular pattern creates a “revolving door” of treatment that increases in both cost and danger as patients are weakened by each hospital stay.

These re-admissions, consequent in-home and nursing home care can add billions to the cost of the disease, meaning the astronomical costs we’ve calculated aren’t even telling the whole story.

What does the future look like?

Some steps have already been taken to improve the outlook for minimizing sepsis damage. In 2012, New York state passed regulations requiring every hospital to have a plan that identifies sepsis and provides prompt treatment. Beginning in 2017, the Centers for Medicare & Medicaid services (CMS) will adjust hospital payments based on quality of sepsis care—hospitals deemed as having high-quality care will receive more money.

Raising public awareness of the symptoms and dangers of sepsis is a top priority for groups such as the Sepsis Alliance, who hope to lower rates of the disease. The earlier a patient suspects sepsis and gets checked out, the easier the condition is to treat, and the lower the risk of long-term complications.

Doctors, policymakers, and researchers are also working to improve the care of sepsis patients once they’re in the hospital. Physician groups have been at work on a prediction tool called qSOFA, which would identify which infection patients are at risk for prolonged-care or death. Researchers have also found some data suggesting the risk for sepsis is increased when healthy bacteria are disturbed, offering the possibility that supplements or diet change could reduce the risk for this serious condition.