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Cracking the Flu Code: Influenza, an In-Depth Guide

Dreaded during the cooler months, the flu, a highly contagious respiratory infection caused by a virus, spreads easily through droplets or contaminated surfaces. Annually, it afflicts a billion people, leading to 5 million serious illnesses and 650,000 deaths, according to the World Health Organization. Recognizing symptoms and practicing preventive measures is crucial for overall health, especially for those with underlying health conditions.

What causes influenza?

Influenza is caused by a virus in the prthomyxovirus family. It is a microscopic infectious entity that requires a living host to multiply and survive. Some viruses, like influenza, can survive outside the host for 24–48 hours on hard surfaces. Influenza is an easily adaptable virus that mutates frequently. Because of its ability to change rapidly, this impacts a person's ability to fight infection. Annual vaccination exists and is recommended because of the flu's variability. This variability affects:

  • How contagious the virus is (its infectivity)
  • How severe of an infection one develops (virulence)
  • Species that can be affected
  • Ability of the virus to cause a pandemic

Influenza types

Influenza comes in four different varieties called types. These include influenza A, B, C, and D.

  • Influenza A. Can infect numerous species, including birds, pigs, dogs, and people. Some of these are species-specific, while others can infect numerous species. These are broken down into subtypes based on viral surface proteins.
  • Influenza B. This group of viruses occurs only in humans.
  • Influenza C. This group usually infects pigs and humans and may cause mild symptoms in people.
  • Influenza D. The primary species affected by this type is cattle. However, it can spill into other species, but is not known to infect people.

Flu season generally ranges from October to May in the Northern Hemisphere. Influenza A and B account for most cases during this time of year.

Influenza A

Influenza A types are divided further according to subtype based on two viral surface proteins. The two flu-associated proteins are hemagglutinin (H) and neuraminidase (N). They act as antigens (substances that a body's immune system will react to and mount a response by creating an antibody) within a host's body.

The H and N of flu A can change easily, leading to a different virus that one's immune system may not fully recognize. This plays a role in why vaccination is required annually. There are 18 H subtypes and 11 different N subtypes. These can combine (or reassort) into numerous combinations, which could be unique and lead to novel viruses. The subtypes of flu circulating vary regionally and each year.

Influenza B

Influenza B viruses are another type that only infects humans. This group is divided into two lineages known as B/Yamagata and B/Victoria. They are then divided further into clades and sub-clades, as are influenza A viruses. The lineages circulating will vary geographically and from year to year.

Difference between flu A and B

The terminology and nuances between influenza A and B can confuse many. The differences include species at risk of infection, host species, severity of disease, and risk of triggering a pandemic. Whether you have influenza A vs. B, while interesting to researchers, statisticians, and public health professionals, symptoms, treatment, and ways to prevent spread are the same.

The difference between influenza A and B isn't an easily answered question. Both contribute to seasonal flu, and both can cause severe symptoms. However, pandemics have only been associated with influenza A.

Canine influenza (dog flu)

You might have heard of an increase in respiratory cases among dogs across various states in the continental U.S. during the late fall and winter of 2023–2024. No specific virus or bacteria has been determined to be the cause. However, one commonly seen virus that induces flu symptoms in dogs — such as coughing, thick snotty discharge, weakness/lethargy, fever, and eye discharge — and may lead to pneumonia or worse, includes canine influenza. Canine influenza primarily affects dogs, and the culprit is influenza A subtypes H3N8 or H3N2. These subtypes have not been shown to infect people.

Dogs at risk of canine influenza include those who board, participate in doggy daycare, are groomed regularly, or are in shelters or other communal situations. As with human flu, it is very contagious, spreading easily from one dog to another. Talk to your veterinarian about whether your pet should be vaccinated against canine influenza.

Who’s at risk for flu?

People of any age can get influenza. Many people are able to fight off infection or develop only mild, short-lived symptoms. Unfortunately, some people are not able to do it as easily. High-risk patients include:

  • Children under 5, especially infants under 6 months
  • Adults over 65
  • People with chronic illnesses like lung disease, heart disease, or diabetes
  • Pregnant people
  • People with weakened immune systems
  • People with a body mass index (BMI) of 40 or more (obesity)
  • People living in long-term care or residential care facilities

Those at a higher risk of developing influenza should take precautions, ensure to practice proper hygiene, stay clear of those who show signs of respiratory disease (even if not the flu), get vaccinated annually, and if symptoms develop, seek care early on before complications take hold.

Flu transmission

Flu transmission happens all too easily. It occurs via droplets spread through the air when someone with the flu coughs, talks, or sneezes, and it enters through another person's mouth, nose, or eyes. Individuals within six feet of an infected person are at greatest risk of exposure. Further, contamination of surfaces such as tables, couches, door knobs, clothing, and other materials provides another source of infection, as the virus can survive on these surfaces long enough to pass to another person.

Symptoms of influenza

Symptoms of influenza A or B range from none to severe and begin within 1–4 days of exposure, generally lasting 1–7 days in most individuals. However, some people may be sick for longer.

Influenza affects the nose, sinuses, throat, and lungs. Symptoms may include:

  • Fevers
  • Chills
  • Runny or stuffy nose
  • Sore throat
  • Dry cough
  • Shortness of breath
  • Headache
  • Body aches
  • Generalized weakness
  • Fatigue/tiredness
  • Watery eyes
  • Loss of apetite

Though vomiting and diarrhea are not common flu symptoms, diarrhea may occur, primarily in small children, due to sinus drainage. This is uncommon in adults or older children. If you also develop vomiting with your respiratory symptoms and other flu-like signs, you may have another viral infection or a complication, and you should seek professional medical advice.

If you experience a loss of smell or taste, this is not a symptom associated with the flu, and you should consider testing for COVID-19.

Diagnosing influenza

The number of people who get flu annually is likely underestimated because many people don't see their doctors or get tested. However, your healthcare provider can diagnose influenza via physical exam, symptoms, and various testing methods if necessary. Depending on the test used, one may get only a positive or negative result, while others may determine the strain (A vs. B).

Normally, the test involves a deep nasal swab; results can be received in as little as 15 minutes. Testing may be done if you are high-risk or are concerned about your symptoms. In the post-COVID-19 world, testing may also be done to differentiate influenza from COVID and other respiratory viruses of concern. Both diseases have similar symptoms and can potentially lead to long-term complications. A diagnosis may help determine how long one should stay out of work, away from those at high risk of illness, and help with future healthcare needs.

Managing the flu

In most cases, treatment for flu is unnecessary in otherwise healthy people. The virus will run its course in a few days (1–7 generally) but sometimes lasts longer. People are advised to drink plenty of fluids to stay hydrated, get plenty of rest, and treat symptoms as needed. Treatment can consist of home remedies to soothe coughs, sinus congestion, and sore throats, though some people may require over-the-counter medications to lessen symptoms and find relief.

Why no antibiotics?

Antibiotics do not treat the flu — they treat bacterial diseases. Unlike bacteria, viruses are not living organisms outside a host and do not respond to antibiotics. Giving antibiotics unnecessarily can lead to many problems, including GI upset, damage to the healthy bacteria in one's GI tract, and an increased risk of developing antimicrobial-resistant organisms (bugs that can resist treatment in the future). Thus, please don't go to your doctor insisting that you need antibiotics because you have flu-like symptoms. Most of the time, our respiratory symptoms are due to viruses (or allergies).

Antivirals may be helpful

Antiviral medications, such as Tamilflu®, have been developed to treat some viruses by preventing them from reproducing and reducing symptoms. These medicines do not treat the symptoms caused by the virus and might only decrease the overall length of time that the virus stays in the body. These drugs are not for everyone, may interact with medications taken daily for chronic diseases, and may cause side effects themselves.

While most people do not require antivirals, if you are at high risk for complications from illness and feel you would benefit from antiviral therapy, contact a medical professional, ideally within 48 hours of symptom onset. For high-risk patients, research suggests that the optimal time to start is within two days of getting sick. Those without underlying diseases, those vaccinated, and the general population often do not need these meds and will recover with time, patience, sufficient sleep, taking it easy, and supportive care.

Antiviral drugs may be beneficial for flu, but they only shorten the course of disease; they won't stop symptoms or cure you of the virus.

Annual flu vaccination

Healthcare providers strongly encourage patients to consider yearly influenza vaccinations to prevent becoming ill from the flu. This is especially true for those who are high-risk, including those who are pregnant, immunocompromised, have respiratory diseases such as asthma, have heart disease, or have diabetes. Contact your healthcare provider to discuss the best time to get vaccines and any possible risks or complications.

  • Vaccines are available for anyone 6 months of age and older. Remember, however, that vaccinations do not guarantee you will not be infected (contract the virus) but may prevent the onset of symptoms. For those who do get sick, vaccinations have been shown to decrease symptom severity and disease length.
  • Vaccines help the body mount an immune response to influenza. While our bodies can develop antibodies as a natural defense against the virus when exposed, these antibodies may not be able to fight off new strains of the virus, and with passing time, these antibodies will decrease, increasing one's risk of illness. Thus, vaccinations help boost the immune system and improve chances of preventing illness or developing only mild symptoms.

A variety of flu vaccines are produced each year with different methods of action and age groups in mind. For those who have an egg allergy, there is an egg-free option. Talk to your healthcare provider to determine which vaccine is right for you.

Complications of influenza

For those who are high-risk, influenza poses a threat they may not be able to fight. The virus can weaken them, making it harder for their bodies to recover. These individuals are at higher risk for complications. However, complications can occur in anyone. Influenza may result in:

  • Pneumonia
  • Worsening of chronic underlying diseases (e.g., asthma, COPD, heart disease, or diabetes)
  • Sinus infections
  • Ear infections
  • In addition to long-COVID symptoms, influenza may also trigger long-term complications

Controlling the spread

Due to the highly contagious nature of the virus, it is important to prevent spreading it if you or anyone around you has it. Even if you show no symptoms, you could be infected and spread the virus. Prevent the spreading with the following steps:

  1. Frequently wash hands with warm soap and water or use a 60% or higher alcohol hand sanitizer.
  2. Do not touch your nose, mouth, or eyes.
  3. Stay home. Do not go out to public places when you have symptoms or have been in contact with someone who does.
  4. If you must go out when sick, wear a facemask to protect others.
  5. Avoid being around other people who have symptoms as much as possible.
  6. Clean surfaces that are frequently touched like tables, doorknobs, phones, computers, etc.
  7. Ensure you remain well hydrated.
  8. Eat a healthy, well-balanced diet.
  9. Get plenty of sleep.

Combating flu

Unfortunately, respiratory viruses affect us all, from COVID-19 to influenza to the common cold. Prevention is key and accomplished through annual vaccinations, proper hygiene practices, maintaining a healthy immune system, and avoiding those with obvious signs of illness.

Flu is a very contagious disease; if you are concerned you may have it, protect others. Seek medical care if your symptoms are not improving in 1–4 days or are worsening, and over-the-counter medications and home remedies aren't improving your symptoms.

While this illness can make you feel poorly, most otherwise healthy people can return to normal life within a few days to a week. Taking adequate measures to prevent the spread of influenza is instrumental in reducing the number of people exposed and becoming ill. Taking precautions may help prevent the next flu pandemic one day.


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