COVID-19 is typically signaled by three symptoms: a fever, an insistent cough and shortness of breath. But older adults — the age group most at risk of severe complications or death from this condition ― may have none of these characteristics.

Instead, seniors may seem “off” — not acting like themselves ― early on after being infected by the coronavirus. They may sleep more than usual or stop eating. They may seem unusually apathetic or confused, losing orientation to their surroundings. They may become dizzy and fall. Sometimes, seniors stop speaking or simply collapse.

“With a lot of conditions, older adults don’t present in a typical way, and we’re seeing that with COVID-19 as well,” said Dr. Camille Vaughan, section chief of geriatrics and gerontology at Emory University.

The reason has to do with how older bodies respond to illness and infection.

At advanced ages, “someone’s immune response may be blunted and their ability to regulate temperature may be altered,” said Dr. Joseph Ouslander, a professor of geriatric medicine at Florida Atlantic University’s Schmidt College of Medicine.

“Underlying chronic illnesses can mask or interfere with signs of infection,” he said. “Some older people, whether from age-related changes or previous neurologic issues such as a stroke, may have altered cough reflexes. Others with cognitive impairment may not be able to communicate their symptoms.”

Recognizing danger signs is important: If early signs of COVID-19 are missed, seniors may deteriorate before getting needed care. And people may go in and out of their homes without adequate protective measures, risking the spread of infection.

Dr. Quratulain Syed, an Atlanta geriatrician, describes a man in his 80s whom she treated in mid-March. Over a period of days, this patient, who had heart disease, diabetes and moderate cognitive impairment, stopped walking and became incontinent and profoundly lethargic. But he didn’t have a fever or a cough. His only respiratory symptom: sneezing off and on.

The man’s elderly spouse called 911 twice. Both times, paramedics checked his vital signs and declared he was OK. After another worried call from the overwhelmed spouse, Syed insisted the patient be taken to the hospital, where he tested positive for COVID-19.

“I was quite concerned about the paramedics and health aides who’d been in the house and who hadn’t used PPE [personal protective equipment],” Syed said.

Dr. Sam Torbati, medical director of the Ruth and Harry Roman Emergency Department at Cedars-Sinai Medical Center, describes treating seniors who initially appear to be trauma patients but are found to have COVID-19.

“They get weak and dehydrated,” he said, “and when they stand to walk, they collapse and injure themselves badly.”

Torbati has seen older adults who are profoundly disoriented and unable to speak and who appear at first to have suffered strokes.

“When we test them, we discover that what’s producing these changes is a central nervous system effect of coronavirus,” he said.

Dr. Laura Perry, an assistant professor of medicine at the University of California-San Francisco, saw a patient like this several weeks ago. The woman, in her 80s, had what seemed to be a cold before becoming very confused. In the hospital, she couldn’t identify where she was or stay awake during an examination. Perry diagnosed hypoactive delirium, an altered mental state in which people become inactive and drowsy. The patient tested positive for coronavirus and is still in the ICU.

Dr. Anthony Perry, an associate professor of geriatric medicine at Rush University Medical Center in Chicago, tells of an 81-year-old woman with nausea, vomiting and diarrhea who tested positive for COVID-19 in the emergency room. After receiving IV fluids, oxygen and medication for her intestinal upset, she returned home after two days and is doing well.

Another 80-year-old Rush patient with similar symptoms — nausea and vomiting, but no cough, fever or shortness of breath ― is in intensive care after getting a positive COVID-19 test and due to be put on a ventilator. The difference? This patient is frail with “a lot of cardiovascular disease,” Perry said. Other than that, it’s not yet clear why some older patients do well while others do not.

So far, reports of cases like these have been anecdotal. But a few physicians are trying to gather more systematic information.

In Switzerland, Dr, Sylvain Nguyen, a geriatrician at the University of Lausanne Hospital Center, put together a list of typical and atypical symptoms in older COVID-19 patients for a paper to be published in the Revue Médicale Suisse. Included on the atypical list are changes in a patient’s usual status, delirium, falls, fatigue, lethargy, low blood pressure, painful swallowing, fainting, diarrhea, nausea, vomiting, abdominal pain and the loss of smell and taste.

Data comes from hospitals and nursing homes in Switzerland, Italy and France, Nguyen said in an email.

On the front lines, physicians need to make sure they carefully assess an older patient’s symptoms.

“While we have to have a high suspicion of COVID-19 because it’s so dangerous in the older population, there are many other things to consider,” said Dr. Kathleen Unroe, a geriatrician at Indiana University’s School of Medicine.

Seniors may also do poorly because their routines have changed. In nursing homes and most assisted living centers, activities have stopped and “residents are going to get weaker and more deconditioned because they’re not walking to and from the dining hall,” she said.

At home, isolated seniors may not be getting as much help with medication management or other essential needs from family members who are keeping their distance, other experts suggested. Or they may have become apathetic or depressed.

“I’d want to know ‘What’s the potential this person has had an exposure [to the coronavirus], especially in the last two weeks?’” said Vaughan of Emory. “Do they have home health personnel coming in? Have they gotten together with other family members? Are chronic conditions being controlled? Is there another diagnosis that seems more likely?”

“Someone may be just having a bad day. But if they’re not themselves for a couple of days, absolutely reach out to a primary care doctor or a local health system hotline to see if they meet the threshold for [coronavirus] testing,” Vaughan advised. “Be persistent. If you get a ‘no’ the first time and things aren’t improving, call back and ask again.”