The coronavirus is most brutal to the old and the chronically ailing. That is a vast cohort in the United States — millions of people — who are not blessed with youth and good health and who now face an enigmatic pathogen that no human immune system has ever encountered and for which there is no vaccine.

Late Monday, the Centers for Disease Control and Prevention released new coronavirus guidelines that attempted to describe who is at higher risk and provide common-sense counsel on how everyone can protect themselves and their families and communities from covid-19, the flu-like disease caused by the novel coronavirus.

The CDC stated that “older adults” are at elevated risk, but it did not define that phrase. It said those suffering from “serious chronic medical conditions,” such as heart disease, diabetes and lung disease, are also at greater risk.

People over the age of 80 with one of these chronic diseases are at the highest risk and should take precautionary steps, such as avoiding crowds and nonessential travel, officials advised. Stay close to home, stockpile essential supplies and medicines, and figure out a backup plan if a caregiver becomes too sick to provide care.

“My parents are in their 80s,” Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases, said on a teleconference Monday. “They’re not in an area where there is currently community transmission. But I’ve asked them to stick close to home.”

The new CDC recommendations include familiar advice about vigorous and frequent hand-washing and avoiding touching one’s face. But they also included other suggestions, such as avoiding touching surfaces like elevator buttons in places where many people gather. “To the extent possible, avoid touching high-touch surfaces in public places — elevator buttons, door handles, handrails, handshaking with people, etc. Use a tissue or your sleeve to cover your hand or finger if you must touch something.”

Although covid-19 has often been compared to the flu, which is also a respiratory infection, data from China shows that it has a significantly higher case fatality rate than seasonal flu. A recent World Health Organization report by an international team of researchers, including from China, found that the case fatality rate for covid-19 patients over the age of 80 in China was 21.9 percent.

Few children have been seriously ill from covid-19, and most adults who get infected will suffer only mild or moderate symptoms, according to infectious disease experts. But more serious outcomes have been seen among people over the age of 60. More than 4,000 people, most of them in China, have died so far. Many of those had underlying health problems.

Patients of all ages with no underlying chronic conditions had a fatality rate of 1.4 percent, according to the WHO report. Covid-19 patients with cardiovascular disease had a rate of 13.2 percent; with diabetes, 9.2 percent; with hypertension, 8.4 percent; with chronic respiratory disease, 8 percent; and with cancer, 7.6 percent.

How the disease will impact the United States remains to be seen. Air pollution in big Chinese cities such as Wuhan, where the outbreak began, could be a factor in the death rate there. So could the high rates of smoking in China, particularly among men, who were more likely than women to die of covid-19.

But the United States has its own health challenges. Life expectancy at birth has flattened out over the past decade. Research shows a rise in death rates among young and middle-aged adults.

The U.S. public is remarkably unhealthy compared to people in similarly affluent nations — a phenomenon known as the U.S. health disadvantage. One example: A 2015 CDC study found that 30 million Americans have diabetes.

“People don’t realize how bad the health of the American population is,” said Eileen Crimmins, a professor of gerontology at the University of Southern California. “When we compare our health to Europeans, we have more disease, more disability.”

She said the 2016 Health and Retirement Study found that 38 percent of people between the ages of 50 and 59 had at least one of four diseases: diabetes, cancer, heart disease or lung disease. That percentage increases with each decade, peaking at 70 percent for people between 80 and 89.

Many Americans now have two or more chronic diseases — a phenomenon that has increased in recent decades. According to the CDC, 21 percent of people between the ages of 45 and 64 had at least two chronic diseases in a survey conducted in 2009-2010, compared with 16 percent in 1999-2000.

One explanation for the elderly’s higher mortality is that as people age, their immune systems age, too, in the same way that bones get weaker. Cell-mediated immunity is an extremely complex process, and the various elements do not synchronize as well in older people, said Wilbur Chen, a University of Maryland infectious disease physician.

“It’s like an orchestra that’s out of tune,” Chen said. “One section is not playing. The other is playing too loud. One is out of rhythm. One is playing two beats; the other is playing three beats. It totally sounds wrong.”

Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, said overreaction of the immune system in some cases wreaks havoc on blood vessels.

“You can’t keep the blood in your circulatory system,” he said. “It’s leaking out. With the dilation of the blood vessels, it’s like a hose that’s got holes in it. If you keep giving them more fluid, you drown the lungs.”

Among cancer patients, people with blood malignancies such as non-Hodgkin’s lymphoma, multiple myeloma and acute lymphoblastic leukemia are at highest risk for coronavirus complications, said Steve Pergam, an infectious disease specialist at the Fred Hutchinson Cancer Research Center in Seattle. Those diseases affect the body’s infection-fighting cells. The treatments for those illnesses — such as bone-marrow transplants and chemotherapy — further suppress the immune system, he said.

Patients under treatment for other malignancies also are at elevated risk. While chemotherapy has the biggest immunosuppressive effects, radiation and surgery also can inhibit the body’s disease-fighting ability, doctors say.

Cancer patients face another problem — fear. And that can be even more damaging.

Jalal Baig, a Chicago oncologist, said three of his patients have told him recently they are considering stopping chemotherapy because they are concerned about contracting the virus. He said only one — a woman with breast cancer — has actually followed through, against his advice.

“It’s so frustrating,” Baig said, adding that the patient has a good chance of being cured by the chemotherapy. “Allowing cancer to progress is probably more harmful over the long term.”

Steven Nissen, a cardiologist at the Cleveland Clinic, said the higher mortality rate for virus-stricken heart patients probably reflects the buildup of fluid in the lungs, which taxes the heart.

“If you flood the lungs with fluid, the workload on the heart goes up,” he said. The pulmonary problem “overloads the heart, and the heart fails, and people get into trouble.”

He told one patient on Monday to forgo a cruise and others to avoid large crowds and make sure they have extra medication in case they have to isolate themselves.

Ann Partridge, a breast cancer specialist at the Dana-Farber Cancer Institute, said she is trying to see healthy cancer survivors for routine follow-ups now so they don’t have to come to the hospital if the outbreak worsens. To protect both vulnerable patients and health care workers, the cancer center also is exploring ways to do more telemedicine. And it has limited patients to bringing just a single companion to reduce the chance of infection from people who might have the virus but not be diagnosed.

“It’s important not to overwhelm the health-care system,” she said. “We need to be available to the people who need us.”