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You'll have heard that the flu can go from a nuisance to dead serious by causing complications, but what exactly can happen as the result of the flu? Let's take a look at the life-threateningly serious conditions that can arise from influenza.

You may think of the flu as an annoying but relatively mild and harmless respiratory infection, but the truth is that influenza can turn deadly. Since 10 to 20 percent of the global population are struck by flu each year (and you may be one of them, especially if you don't get a flu shot), it's important to be aware of the serious complications that you can fall victim to if you catch a flu.

When flu complications are mentioned, it's usually in a general "complications such as pneumonia" sense — but what can the flu really do? Let's take a closer look. 

1. How the flu can affect your heart: Possible cardiovascular complications

Heart disease and the flu have long been thought to be linked, as cases of each go up during flu season. Though fairly rare, cardiovascular complications of the flu can be extremely serious, and though existing heart disease patients are, of course, at a greater risk of suffering cardiovascular complications, the flu can also trigger them in people who weren't already diagnosed with heart disease.

The cardiovascular complications of the flu include:

  • Myocarditis, an inflammation of the heart that can strike when a virus gets into this vital organ, causing symptoms like chest pain, pale and cold hands and feet, faster breathing and a faster heart rate, and swelling of the legs in addition to the usual flu symptoms like fever and fatigue. (Some people make a complete recovery, but long-term heart failure is also possible. Myocarditis is one of the reasons you'll often see articles about the flu mention that anyone who experiences these symptoms when they have or think they have the flu should seek immediate medical attention! It's rare though, affecting anything from 0.4 to 13 percent of people who were diagnosed with influenza.)
  • The flu may also trigger ischemic heart disease, a kind of heart disease in which the arteries fail to get enough oxygen-rich blood to your heart — something that's thought to be the result of the inflammation associated with the flu. The warning signs are similar to those of myocarditis. 
  • While rare, influenza has additionally been associated with an increased risk of stroke (which is also considered a cardiovascular disease). Symptoms include (usually one-sided) numbness/paralysis in the face, an arm, or a leg, confusion, finding it hard to talk, and vision issues. 

2. The flu can trigger neurological conditions

You've already have heard of some of the possible neurological complications of the flu:

  • Encephalitis is an inflammation of the brain. Most often caused by viruses, including the flu, it's nonetheless a rare condition. Besides fever, a headache, not feeling like eating, and being low on energy (all things the flu itself can cause), red flags include confusion, irritability, clumsiness, and sensitivity to light. Some people will experience a stiff neck as well. Though some patients recover, others can suffer permanent brain damage. Encephalitis can even be fatal. 
  • Guillain‐Barre syndrome is an autoimmune neurological disorder that most often develops as people are recovering from some kind of infection — of all the infections known to sometimes trigger GBS, influenza sits near the top of the list. This rare condition can cause tingling sensations that often start off in the legs, muscle weakness, issues with vision, clumsiness, and changes in the patient's heart rate and blood pressure, among other things. GBS only affects a few thousand people a year in the US and though the majority of patients do make a full recovery, it's something to be aware of. 
  • Narcolepsy, a neurological condition that's quite well-known because it can make people fall asleep during the day as they engage in their normal activities, has also been associated with the flu. 
  • Then there's Reye's syndrome, which has been linked to the flu — but which is more strongly associated with giving aspirin to children, something parents would once have considered normal if their children were battling influenza. You can reduce your child's risk of Reye's syndrome greatly simply by opting for another, much safer, pain killer and fever reducer like acetaminophen (Tylenol, paracatamol). 

3. Rhabdomyolysis, a muscular complication of the flu

Rhabdomyolysis is a condition in which muscle fibers "die off" and are then expelled into the bloodstream. It often follows injury, but fevers can also trigger this rare but serious condition, which can in turn cause its own complications, including kidney failure. Symptoms to watch out for include:

  • Not peeing as often or as much, and when you do urinate, the color will be very dark — brown or red. 
  • Muscle stiffness, weakness, and pain
  • Generally feeling very weak. 

People with rhabdomyolysis may also experience seizures. Though some people recover fully, your prognosis will depend on the extent of any damage that was done to your kidneys. 

4. Acute kidney disease as the result of influenza

It's not quite clear whether the flu can cause acute kidney disease, or whether influenza can simply make existing kidney disease worse. In patients hospitalized for the flu who needed to be in the intensive care unit, however, research has found that somewhere between 18 and 66 percent had acute kidney disease. (Keep in mind that this absolutely doesn't mean your risk is that high if you just got the flu and didn't need to be hospitalized, or even if you were hospitalized but not in the ICU!)

5. Liver disease because of the flu?

While the flu is extra dangerous for people with liver disease — as it is for people with many chronic conditions — studies also point to the idea that influenza can cause liver disease. This one requires further study before it's clear just how often and under what circumstances it happens. 

6. Sepsis and influenza

Sepsis, an extreme reaction to infection that causes confusion, low blood pressure, and fast breathing and that can become fatal if it progresses to septic shock, can result from the flu. As with many other complications, it's more common in the very young or old, pregnant women, and those with chronic health conditions — people with weaker immune systems, in other words. Sepsis may lead to discolored skin, urinating less often, breathing problems, chills, a high fever, and feelings of extreme weakness. In come cases, people will lose consciousness. 

7. Pneumonia: The most-talked about flu complication

Let's finish this off with pneumonia, one of the more talked-about complications of the flu in which the alveoli (air sacs) become clogged with pus or fluids. Pneumonia can be caused by bacteria and fungi, but also viruses — and any respiratory infection can turn into pneumonia, including the flu. 

The deceptive thing about pneumonia is that its initial symptoms really aren't that different than those you'll have from the flu itself — you'll cough, expel mucus, have a fever, and experience chills. Trouble breathing and chest pain are the symptoms that differentiate pneumonia from your initial infection, and that alert you that it's high time to see a doctor. 

It's important to be aware of the possibility that the flu can lead to pneumonia, and that vulnerable groups like the elderly and people with lung disease such as asthma and COPD are at a higher risk. There are vaccines that greatly reduce your risk of some types of pneumonia; ask your doctor if you are a good candidate for these pneumococcal vaccines. 

Preventing the flu means preventing flu complications

We promised you "serious complications", so we talked about those — we didn't even include less serious complications, like sinus infections and conjunctivitis. They should go a long way toward explaining why the flu is indeed considered a serious respiratory infection.

Thankfully, getting your annual flu shot greatly lowers your risk of catching influenza, and getting vaccinated also reduces the duration and severity of symptoms should you still develop it. The CDC recommends that everyone over the age of six months receives a flu vaccine each year, but this is especially important for people who belong to vulnerable groups. That's children, pregnant women, the elderly, and people with chronic health conditions; in addition to the complications mentioned here, influenza can also exacerbate the symptoms of existing conditions, sometimes with extremely dangerous consequences. 

Should you still end up with flu-like symptoms, see your doctor for diagnosis — if you do have the flu, prescription antivirals can shorten your illness, reduce your symptoms, and lower your risk of complications. Whether or not you follow this advice, always seek medical advice immediately if you (think you) have the flu and notice:

  • Breathing difficulties
  • Chest pain
  • Confusion
  • Dizziness
  • Extreme weakness
  • Very intense muscle pain
  • A lack of urination
  • New symptoms right as you thought you were recovering from the flu
  • That symptoms of existing conditions have gotten worse

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