Between one million and three million Americans are given diagnoses of sepsis each year, and 15 percent to 30 percent of them will die, Dr. Frieden said. Sepsis most commonly affects people over 65, but children are also susceptible. According to one estimate, more than 42,000 children develop sepsis in the United States every year, and 4,400 die.

Sepsis develops when the body mounts an overwhelming attack against an infection that can cause inflammation in the entire body. When that happens, the body undergoes a cascade of changes, including blood clots and leaky blood vessels that impede blood flow to organs. Blood pressure drops, multiple organs can fail, the heart is affected, and death can result.

“Your body has an army to fight infections,” said Dr. Jim O’Brien, the chairman of Sepsis Alliance. “With sepsis, your body starts suffering from friendly fire.”

Sepsis appears to be rising. The rate of hospitalizations that listed sepsis as the primary illness more than doubled between 2000 and 2008, according to a 2011 C.D.C. study, which attributed the increase to factors like the aging of the population, a rise in antibiotic resistance and, to some extent, better diagnosis.

Sepsis is a contributing factor in up to half of all hospital deaths, but it’s often not listed as the cause of death because it often develops as a complication of another serious underlying disease like cancer. So although death certificates list sepsis as a cause in 146,000 to 159,000 deaths a year, a recent report estimated that it could play a role in as many as 381,000.

Yet advocacy organizations say many Americans have never heard of sepsis and don’t know the signs and symptoms.