Definition

Influenza (flu) is a contagious respiratory illness caused by viruses that infect the nose and throat and can also spread to the lungs. While the flu can resolve on its own, it can lead to mild to severe illness and, in some cases, even death, especially among high-risk groups. In countries with four seasons, the flu most commonly occurs in the winter and early spring.

Causes

Influenza is caused by infection with influenza viruses, which are classified into four types: A, B, C, and D. Types A and B, referred to as human influenza viruses, are responsible for seasonal flu epidemics each year.

Influenza A virus is further categorized into various subtypes based on the surface proteins of the virus and can be found in different animal species. Transmission from animals to humans is possible. Influenza A virus is also the only one capable of causing flu pandemics. Pandemics occur when a variant of the influenza A virus emerges with the ability to infect humans and spread globally. Currently, the subtypes circulating among humans are A(H1N1) and A(H3N2).

Influenza B virus undergoes mutations at a slower rate than type A. Infections with influenza C viruses typically result in mild illness and do not cause widespread epidemics. Influenza D viruses do not infect humans.

The primary mode of transmission for influenza viruses is through small droplets expelled when an infected person coughs, sneezes, or talks. These droplets can reach the mouths or noses of people nearby, typically within a distance of 2 meters. Additionally, transmission can occur through contact with surfaces contaminated with the influenza virus, followed by touching the nose, mouth, or eyes.

Notably, individuals can spread the flu even before symptoms appear. The most contagious period is typically the third and fourth days after symptoms manifest. In healthy adults, virus transmission can occur one day before symptom onset and continue for 5-7 days afterward. Children and individuals with weakened immune systems may transmit the virus for a longer duration. Importantly, individuals without symptoms can also spread the virus, highlighting that the presence or absence of symptoms does not determine transmission.

Risk Factor

While anyone can contract the flu, certain individuals are at a higher risk of experiencing serious complications. These high-risk groups include:

• Elderly individuals aged 65 years or older
• Pregnant women and breastfeeding mothers within two weeks after giving birth
• Children under 5 years old, especially those under 2 years old
• Individuals with certain medical conditions such as asthma, diabetes, weakened immune systems (e.g., HIV/AIDS, cancer, long-term corticosteroid use, teenagers undergoing long-term aspirin therapy), heart disease, stroke, blood disorders (e.g., sickle cell disease), chronic lung disease (e.g., chronic obstructive pulmonary disease), kidney disease, liver disease, metabolic disorders, and obesity
• Individuals living or working in crowded places such as nursing homes, military barracks, or hospitals
• Certain racial or ethnic groups, such as Native Americans, who may be more susceptible to severe flu symptoms

Symptoms

Influenza typically manifests with a range of symptoms that may vary in severity. These symptoms can include:

• Fever (though not all flu patients experience fever)
• Chills
• Cough
• Sore throat
• Stuffy or runny nose
• Muscle aches
• Headache
• Fatigue
• Some individuals may also experience vomiting and diarrhea, which is more common in children

Diagnosis

The doctor will conduct an in-depth interview with you about the issues you're experiencing, your family's medical history, and the therapy you have received. Apart from that, the doctor will perform a physical examination, particularly of your respiratory tract.

In most cases, clinical symptoms alone can be used to diagnose influenza, especially during an epidemic or flu season. However, in rare cases, your doctor might recommend that you take additional tests. Several tests are available to diagnose influenza, including:

Rapid Influenza Diagnostic Tests (RIDTs)

Rapid influenza diagnostic tests (RITDs) are one of the available tests. These tests detect influenza virus antigens and provide results within 10–15 minutes. However, they may not be as accurate, and a negative result does not definitively rule out influenza.

Rapid Molecular Assays

These tests detect genetic material from the flu virus and offer more accurate results within 15-20 minutes.

PCR Testing

Polymerase Chain Reaction tests are highly sensitive and can identify the specific strain or type of influenza virus present. However, results may take several days to obtain.

Chest X-ray

Imaging examinations of the chest area may be recommended to rule out lung infections or pneumonia and assess lung health and surrounding tissues.

Management

For most healthy individuals without underlying health conditions, influenza tends to be a mild illness that resolves on its own. In such cases, antiviral medications are typically unnecessary. Rest and ample fluid intake are sufficient to manage mild flu symptoms. Physicians may recommend over-the-counter medications to alleviate specific symptoms such as fever or pain.

However, for individuals with severe infections or those at high risk of complications, antiviral medications may be prescribed. Distinct from antibiotics, antivirals help alleviate symptoms, shorten the duration of illness, and mitigate the risk of serious complications associated with the flu.

Studies indicate that antiviral drugs offer optimal benefits when initiated within 48 hours of symptom onset. Nonetheless, starting antiviral treatment beyond this window can still be beneficial, particularly for high-risk individuals or those severely affected by the flu.

Complications

While influenza symptoms are typically mild and resolve within 1-2 weeks for healthy individuals and young people, serious complications can arise, particularly among high-risk groups such as children, pregnant women, and individuals with underlying medical conditions. These complications may include:

• Bacterial pneumonia
• Ear infections
• Sinus infections
• Exacerbation of pre-existing medical conditions such as heart failure, asthma, chronic obstructive pulmonary disease (COPD), or diabetes
• Acute respiratory distress syndrome (ARDS)
• Myocarditis (inflammation of the heart muscle)
• Multiple organ failure

Prevention

The most effective way to prevent or reduce the risk of contracting the flu and its severe complications is through annual vaccination. Seasonal flu vaccines are specifically designed to safeguard against influenza viruses responsible for epidemics, encompassing variants of influenza A (H1N1 and H3N2) as well as influenza B viruses. While these vaccines can shield against viruses with similar antigens, they do not provide protection against types C and D influenza.

The benefits of flu vaccination are numerous, including lowering the incidence of flu-related illnesses, mitigating the risk of severe complications, and reducing doctor visits. Ultimately, vaccination can help decrease hospitalizations and even fatalities.

It's recommended that everyone aged 6 months or older receive the flu vaccine annually, particularly those susceptible to complications from influenza. However, certain individuals may not be suitable candidates for vaccination based on factors such as age, medical conditions, and allergies, which should be determined by a healthcare provider. Additionally, individuals who care for those at high risk, such as healthcare workers and nursing home staff, should also receive the flu vaccine.

Infants under 6 months old are highly vulnerable to severe flu illness but are too young to be vaccinated. Thus, it's crucial for caregivers to be vaccinated to protect the infant.

Incorporating daily preventive measures is also essential, including:

• Maintaining distance from individuals displaying symptoms of illness
• Limiting physical contact with others if you're sick to prevent viral transmission
• Covering your mouth and nose with a tissue when coughing or sneezing and promptly disposing of used tissues
• Frequent handwashing with soap and water; when unavailable, using alcohol-based hand sanitizer
• Refrain from touching your eyes, nose, and mouth, as these are entry points for viruses
• Regularly cleaning and disinfecting surfaces and objects that may harbor viruses
• The CDC advises individuals with the flu to remain at home until at least 24 hours after their fever subsides.

This practice not only applies to influenza but also to other respiratory infections.

When to See a Doctor?

If you belong to a high-risk group for flu complications and develop flu-like symptoms, it's advisable to consult a healthcare professional promptly to explore antiviral treatment options if necessary. Adhering to the prescribed medication regimen is crucial for effective management of the flu.