While they may not be hospitalized for respiratory illnesses, “we are seeing some exacerbation of other conditions in individuals who are vulnerable,” said Dr. Sandra Nelson, an infectious disease specialist at Massachusetts General Hospital and an assistant professor of medicine at Harvard Medical School. In some cases, patients were dehydrated from the effects of a virus infection and came in with kidney failure.

Doctors say that it is not always clear what role Omicron plays, but there is a plausible biological explanation for a virus causing patients to develop systemic issues. “You’re going to see kidneys get worse, etc.,” said Dr. Panagis Galiatsatos, a critical care specialist and assistant professor of medicine at Johns Hopkins Medicine.

At St. Luke’s, where Ms. Perez was admitted, roughly two-thirds of the coronavirus-positive patients in the system’s network had a primary diagnosis of Covid, but an additional 15 to 20 percent were diagnosed with other illnesses, like sepsis or acute kidney failure, that doctors said were clearly related to a virus infection.

“It isn’t an incidental diagnosis,” said Dr. Jeffrey Jahre, an infectious disease specialist who is senior vice president for medical and academic affairs at St. Luke’s University Health Network, which operates 11 hospitals in Pennsylvania and New Jersey.

In some cases, these patients may have had a “smoldering” case of diabetes or hypertension that a Covid case pushed over into serious illness, said Dr. Nicholas Kman, an emergency physician at the Ohio State University Wexner Medical Center in Columbus. In other cases, individuals who had successfully been managing their conditions before becoming infected, like Ms. Perez, are coming in with high blood sugar levels or worrying hypertension.

Others, like transplant or cancer patients — although fully vaccinated — are not able to mount a sufficient immune response to protect themselves from serious disease when they become infected.

Doctors say these admissions — often categorized as “with” Covid rather than “for” Covid — have had significant effects on stressed hospitals. “All those patients add to the surge and the volume,” Dr. Kman said, adding that “one or two extra patients can push a health system over the edge.”