The baby seemed fine when Dr. Thomas R. Frieden left for work that July morning in New York more than 20 years ago. But when he returned home several hours later, his son was pale and blazing hot, limp in his wife’s arms.

“My first thought was that he was dead,” said Dr. Frieden, now the director of the Centers for Disease Control and Prevention. “It was so scary, even for someone trained as a physician, to see how quickly someone who’s healthy can become critically ill.”

Dr. Frieden, an infectious diseases specialist, knew time was of the essence. His hunch, which turned out to be correct, was that his son had developedsepsis, a life-threatening condition triggered by an infection that can very quickly spiral out of control.

He called the child’s pediatrician, and within hours, the infant had received a broad-spectrum antibiotic. He recovered within a few days.

Many patients are not so fortunate. 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 pressuredrops, 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.

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Ciaran and Orlaith Staunton, a Queens couple who lost their son Rory, a sixth-grader, to sepsis in 2012, discovered the condition was not even listed on the A to Z index of the C.D.C.’s website at the time. They met with Dr. Frieden and urged him to use his bully pulpit to educate Americans about the disease.

Now the C.D.C. is starting a major public awareness campaign to make sepsis a household word. The first step is to teach people to seek treatment quickly when a loved one begins to show symptoms of sepsis, which include chills or fever; extreme pain or discomfort; clammy or sweaty skin; confusion or disorientation; shortness of breath; and a high heart rate.

The campaign, which has developed educational fact sheets, encourages lay people to suggest the diagnosis to health care providers who may overlook it. Ask the doctor, “Could it be sepsis?” or say, “I’m worried about sepsis.”

“We want people to be able to recognize sepsis just like they recognize a heart attack or stroke, and know they shouldn’t wait until Thursday when the doctor can see them, but go to the emergency room right away,” said Thomas Heymann, executive director of Sepsis Alliance. The group’s motto is, “Suspect sepsis, save lives.”

When blood pressure drops and chokes blood flow to the body’s organs, a person can develop septic shock. For every hour without antibiotics, the probability of dying goes up by 8 percent, Dr. O’Brien said.

While earlier initiatives have focused on reducing sepsis deaths that developed in hospitals, newer studies suggest most cases start in the community setting, before people are hospitalized. The new campaign was spurred in part by the latest C.D.C. study, which examined the hospital records of 246 adults and 79 children, finding that in nearly 80 percent of cases, sepsis had started when the patient was at home.

The study also shed light on who is most susceptible to sepsis. Though it occurs most often in people over 65, infants under the age of 1 are also susceptible, as are people with chronic diseases like diabetes or immune systems weakened from tobacco use, for example. And healthy people can develop sepsis from an infection that’s not treated properly as well.

The Stauntons’ son died after developing an infection that wasn’t recognized by emergency room doctors who examined him at NYU Langone Medical Center. A new state law passed after his death requires hospitals in New York to screen all patients for sepsis in order to start treatment early.

The new C.D.C. study found sepsis is most often associated with lung, urinary tract, skin and gut or intestinal infections, and that many sepsis patients had visited a doctor or been in a health care setting before developing the infection.

The C.D.C. is aiming efforts at health care providers, urging them to consider sepsis and act quickly, and investing in research to study risk factors. It is also emphasizing prevention through better management of chronic diseases, vaccinations and appropriate use of antibiotics.