Spotting the Signs of the Flu

It isn't always easy to tell the difference between the flu and other respiratory diseases

person sitting on couch wrapped in blanket and blowing their nose Photo: Getty Images

The typical signs of the flu—fever, chills, cough—can make a case of this viral illness tricky to distinguish from other respiratory diseases, such as colds and COVID-19. But the flu can also bring some unexpected symptoms, such as stomach problems and pink eye, that you might not realize are related.

In one 2018 study published in the Journal of the American Geriatrics Society, researchers found that older adults who came to the hospital and tested positive for the flu were less likely to have shown classic signs of flulike illness than younger adults.

With the flu and COVID-19 both circulating, it’s crucial to check in with your doctor about even mild symptoms, in case you need a test for one or both. “If you’re worried about one, you need to be worried about the other,” says Kavitha Prabaker, MD, assistant clinical professor in the division of infectious diseases at the David Geffen School of Medicine at UCLA. Here, experts explain how to spot and treat the flu when it looks a bit unusual.

Less Common Flu Symptoms

Even the early stages of the flu might not look exactly as you expect, says William Schaffner, MD, a professor of preventive medicine and infectious disease at Vanderbilt University Medical Center in Nashville, Tenn.

More on the flu

Classically, a flu infection begins not with respiratory symptoms but with the sudden onset of fever along with some combination of chills, headaches, muscle and joint aches, and generally feeling unwell. Cough, sore throat, or runny nose might not arrive until later, Schaffner says. Here are some other less typical symptoms of the flu.

Gastrointestinal problems: Nausea, diarrhea, and vomiting are more commonly seen in children. But they can occur in older adults as well, Schaffner says. These same signs can also accompany COVID-19.

Conjunctivitis: Many viruses, including the flu, can cause conjunctivitis (pink eye)—tearing, burning, and reddening of the eye. It’s not the most common symptom of the flu. But it can be so bothersome that it might lead you to miss a case of the flu if you pay attention to your eye and overlook your cough, Schaffner says.

Low or no fever: Older adults are less likely to have a fever (or a high fever) than younger adults with the flu. In one 2015 study published in the journal Influenza and Other Respiratory Viruses, out of 184 older adults who were hospitalized with the flu, 22 percent never had a temperature of 99° F or higher. The lack of a high fever—or any fever—doesn’t rule out the possibility of a case of the flu.

Vague symptoms, such as loss of appetite: Sometimes signs of the flu (or other infections) include ambiguous symptoms like losing your appetite or feeling unusually fatigued, particularly in older people. These types of signs, which can also include confusion or disorientation, are sometimes noticed first by a person’s companion or caregiver, Prabaker says. If you lose your appetite or feel abnormally weak, tired, or generally unwell, reach out to your doctor.

Less Common Complications

Bacterial pneumonia is one of the most common severe complications of the flu, but the virus can also affect other parts of the body in serious ways. In a 2020 study of almost 90,000 flu cases published in the journal Annals of Internal Medicine, rates of severe cardiac events, such as heart failure, ranged from 12 percent in adults ages 65 to 74 to almost 18 percent in adults 85 and older. And stroke, kidney disease, and neurological complications can also occur.

These complications aren’t likely to be the first flu symptoms you have, but they are one reason it is important to diagnose a case of the flu early. When treated early with antiviral drugs such as oseltamivir (Tamiflu and generic), the chance of severe disease is lower. The flu shot also lowers your risk of severe effects from the virus. It’s not too late to get your jab.

Editor’s Note: A version of this article also appeared in the January 2022 issue of Consumer Reports On Health.

Catherine Roberts

As a science journalist, my goal is to empower consumers to make informed decisions about health products, practices, and treatments. I aim to investigate what works, what doesn't, and what may be causing actual harm when it comes to people's health. As a civilian, my passions include science fiction, running, Queens, and my cat. Follow me on Twitter: @catharob