How the Flu Can Affect Your Health Even After You Feel Better

The influenza virus itself may only be present for a few days, but for some people, the effects can be long-lasting.

You probably know by now some of the major reasons you should get your flu shot this and every year: tens of thousands die from the flu in the United States every year, including many otherwise healthy children and young adults. And while the shot doesn't offer 100% protection against the virus, it's certainly better than nothing. Even if you do get sick, being vaccinated reduces your risk of getting a severe case of the flu and winding up in the hospital. It also makes it less likely that you'll pass the flu on to others.

But there's another, lesser-known argument for getting the flu shot, and for taking other precautions against influenza, as well: The flu isn't just a health risk for the seven days or so that you're physically sick with the virus—it can also have some lasting effects that could affect your health for weeks, months, or even permanently. Here are some of the ways the flu can be a risk factor for health issues, even after you're feeling better.

Heart Attack and Stroke Risk

A number of studies have linked influenza to an increased risk of heart attack and stroke, and scientists have theorized that the inflammatory response triggered by the flu can fuel the development of atherosclerosis, a contributor to heart and artery disease. In a study published this year in the New England Journal of Medicine (NEJM), Canadian researchers found that the risk of having a heart attack was six times higher during the week after being diagnosed with the flu, compared to the year before or after a flu infection.

Other research has suggested that this association persists past those initial seven days of infection: A 2004 NEJM study found that while the increased risks of heart attack and stroke were both highest in the first three days after diagnosis, the dangers only "gradually fell during the following weeks." And in 2008, researchers reported in the European Heart Journal that the risk of stroke after a flu diagnosis remained elevated for up to three months.

"Most people who have studied this agree that two to four weeks, and maybe even into that second month, there is an increased risk of heart attack and stroke," says William Schaffner, MD, an infectious disease specialist at Vanderbilt University. But the message hasn't yet reached the general public, he adds, or even a lot of physicians. "When I mention this to doctors during continuing education classes, they sit up in their chairs; they've never heard this before."

Secondary Infections

The flu can also do a number on the immune system, which can leave people vulnerable to other illnesses and infections—like pneumonia, for example. And while pneumonia is often referred to as a complication of the flu, it's also not unusual for a person to come down with it once their initial flu symptoms have passed.

"It happens quite often," says Sharon Nachman, MD, chief of the division of Pediatric Infectious Diseases at Stony Brook University School of Medicine. "People feel poorly, then they start to feel better, and all of a sudden they feel poorly again and they wonder why they're not getting better. And actually, it's because you don't still have the flu; you have a new, secondary infection."

Those infections are sometimes bacterial, says Dr. Nachman, which means that antibiotics are likely needed to treat them. It can be normal to feel like you're not at 100% following a bad case of the flu, says Dr. Schaffner—but if you don't feel better after a few weeks, talk to your doctor to rule out something more serious.

Out-of-Whack Test Results

The body has to work hard to fight the flu virus, and it can take time to recover to its pre-flu state. So it shouldn't be surprising that a recent case of influenza can affect the results of blood tests and medical screenings, says Dr. Nachman.

"After most viral illnesses, your white blood cell count is going to be low," she says. Other measurements, like cholesterol levels, could also be outside of their normal ranges. If you're scheduled for any routine testing after a bout with the flu, be sure to mention it to your doctor so he or she knows that it could be a factor in your results.

A Slide Into Disability

For elderly patients, getting the flu could be the first step in a continual downward spiral when it comes to their health and their ability to take care of themselves. Not only do older adults face a higher risk of serious complications and death while they have the flu, but they're also at greater risk of reduced quality of life afterward, says Dr. Schaffner.

Studies have shown that a significant percentage of nursing home residents experience major physical decline following flu-like illnesses, and that heart disease patients hospitalized for the flu required more follow-up care after they were discharged. "When we talk to geriatricians, they all nod their heads and say they've known this for years," says Dr. Schaffner.

"When an infection like the flu puts you in a bed, it's remarkable how much muscle tone you lose every day," he adds. "And if you're already on the edge of frailty, it can send you on the downhill slide, and it's very difficult to get your strength and your confidence back completely."

For this reason, he says, doctors and loved ones should pay close attention to elderly patients after a flu diagnosis. "Make sure they're getting the help they need to get back to their normal routine," he says. "In some cases, they may even need some physical therapy to help them do that."

Lost Strength and Endurance

That loss of muscle tone and strength is especially dangerous for the elderly population, but it's also likely to affect younger flu victims as well, says Dr. Schaffner, to a lesser extent. Exercise after the flu can help you feel better, he adds, but he recommends starting with low-intensity activity like brisk walking. "When I've gone back to the gym after having influenza, I've reduced my weights and my reps for some time before I work my way back up to normal," he says.

Dr. Nachman agrees. "Think about it like a trauma: After they take the cast off from your broken leg, you can't run a mile right away," she says. "And after you've had a really bad viral infection, your body needs time to recuperate—and overdoing it right away can make you feel sicker." Listen to your body, she says, and if you feel short of breath or overly fatigued, scale back for a few days.

These post-flu risks may not be as well known or as well publicized as the more obvious symptoms and immediate complications of influenza itself. But they should serve as even more reason to get vaccinated, say the experts we spoke with, and to hopefully prevent getting infected in the first place. "It makes flu an even nastier virus than we thought," says Dr. Schaffner, "and we thought it was plenty nasty already, even without these lasting effects."

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