Influenza: Symptoms, Causes, and Prevention Tips to Keep You Healthy

Influenza is a respiratory disease caused by the influenza A or B virus, occurring frequently in outbreaks and epidemics worldwide, particularly during the winter months when its transmission peaks. The disease spreads primarily through large-particle droplets generated by sneezing and coughing, as respiratory secretions from infected individuals contain significant quantities of the virus. In immunocompetent adults, the average duration of viral shedding is around five days, but this can extend to ten days or longer in more vulnerable populations, such as children, elderly individuals, patients with chronic illnesses, and those with compromised immune systems.

The onset of influenza is typically abrupt, marked by high-grade fever, muscle pain (myalgia), headache, and a general sense of malaise. These systemic symptoms are often accompanied by respiratory complaints, including a dry, nonproductive cough, sore throat, and nasal discharge. While the respiratory tract is the primary site of infection, influenza can lead to complications affecting other organs, such as the lungs, brain, and heart, resulting in severe outcomes like hospitalization.

Don’t let the flu slow you down—let us provide the care you need to get back to feeling your best!

Prevention remains the cornerstone of influenza management, with annual vaccination being the most effective strategy to reduce the risk of infection and its complications. For those who do become infected, early antiviral treatment, ideally initiated within two days of symptom onset, has been shown to significantly decrease both morbidity and mortality. Given the substantial burden of disease associated with influenza, prompt diagnosis, preventive measures, and timely therapeutic interventions are essential for mitigating its impact.

Introduction

Influenza, commonly known as the flu, is a highly contagious viral infection primarily caused by the influenza A or B virus. It primarily affects the upper respiratory system, including the nose, throat, and bronchi, and occasionally involves the lungs. In more severe cases, other organs like the heart, brain, and muscles can also be affected. Influenza occurs globally and leads to significant morbidity and mortality, manifesting in pandemic, epidemic, or seasonal patterns. Epidemics typically occur annually during autumn and winter in temperate regions, contributing to substantial health and economic burdens.

The virus spreads through respiratory droplets expelled when an infected person coughs, sneezes, or talks, and close contact (within 1 meter) is often required for transmission. While most individuals recover within a few days, influenza can lead to serious complications such as pneumonia, and even death, especially in high-risk groups, including pregnant women, the elderly, and those with weakened immune systems. Common symptoms of influenza include high fever, body aches, headache, severe malaise, dry cough, sore throat, and runny nose. Clinically, influenza is distinct from the common cold due to its more severe systemic symptoms.

One of the unique features of influenza is its ability to cause recurrent epidemics, largely due to its continuous antigenic changes. These mutations allow the virus to evade immunity, leading to frequent outbreaks. Mortality is often driven by pulmonary complications, particularly in vulnerable populations.

Etiology-

Influenza viruses are categorized into four types: A, B, C, and D, with types A and B primarily responsible for human infections. Influenza A is the most variable and widely recognized, capable of causing seasonal epidemics. It is further divided into subtypes based on two surface proteins: hemagglutinin (H) and neuraminidase (N). These proteins play key roles in the virus’s ability to infect host cells and spread. There are 18 known hemagglutinin subtypes (H1 to H18) and 11 neuraminidase subtypes (N1 to N11), which can combine to form different strains like H1N1 or H3N2. This variability allows influenza A to evolve rapidly, resulting in new strains each year and contributing to seasonal flu outbreaks.

Influenza B viruses, though less diverse than influenza A, also contribute to seasonal epidemics. They are classified into two major lineages: Yamagata and Victoria. These lineages circulate independently of influenza A and cause similar symptoms but tend to produce less severe epidemics. Unlike influenza A, influenza B does not have the same level of variation and does not undergo the same type of antigenic shifts that lead to pandemics.

Influenza C causes a much milder illness and is not responsible for widespread outbreaks. It typically results in lower respiratory tract infections and does not undergo the frequent mutations seen in types A and B. Influenza D primarily affects cattle and has not been associated with human disease.

Influenza A viruses are of particular concern because they can mutate and reassort, sometimes acquiring new traits that enable them to jump from animals to humans. This was the case with the 2009 H1N1 pandemic, which originated from a swine flu virus but adapted to spread efficiently between humans. Avian influenza, notably the H5N1 and H7N9 strains, is another example of a virus with the potential to cause pandemics if it evolves to transmit easily between people. These animal-to-human transmission events typically start with a limited number of cases but can result in major health crises if the virus acquires the ability to spread from person to person. This is due to the ability of influenza viruses to undergo antigenic shifts, which can lead to global outbreaks or pandemics.

SIGN AND SYMPTOMS

The symptoms of influenza may include:

• Fever
• Chills
• Body aches
• Cough
• Headache (frontal or retro-orbital)
• Sore throat
• Runny or congested nose
• Red, watery eyes
• Fatigue or feeling worn out
• Vomiting or diarrhoea (typically in children)

Epidemiology

Influenza A was first isolated in 1933, followed by the isolation of Influenza B in 1940. These viruses, particularly Influenza A, play a major role in seasonal influenza epidemics, which occur every year in the winter months. The occurrence of these epidemics, which varies in severity, is influenced by several factors including the predominant strain of the virus, the age groups most affected, and the complications that arise from infection.

The severity and length of influenza seasons can vary greatly, with certain strains having a more significant impact than others. For instance, when H3N2 strains are prevalent, the flu season tends to be more severe, especially for vulnerable populations such as young children and the elderly. The virus is capable of causing a range of complications, including hospitalizations and deaths, and the severity of these outcomes differs each year.

The World Health Organization (WHO) plays a crucial role in monitoring influenza activity globally. Through ongoing virologic surveillance, WHO tracks the circulation of influenza viruses by isolating strains from humans in various geographic regions every month. This surveillance helps to monitor the evolution of the virus and informs vaccine development.

Influenza patterns also vary by region. In temperate regions, influenza activity peaks during the winter months. In the Northern Hemisphere, this typically occurs between October and March, while in the Southern Hemisphere, the peak is observed from April to August. In tropical regions, however, influenza viruses circulate year-round, without a distinct seasonal peak. This global surveillance system is essential for tracking and understanding the dynamics of the influenza virus, and it helps in managing public health responses to seasonal epidemics and potential pandemics.

Pathophysiology

Influenza is an acute viral infection primarily affecting the upper respiratory tract, causing inflammation in the respiratory tree and trachea. Symptoms usually last for seven to ten days, with most healthy individuals recovering without complications. The immune response, including the production of interferons, contributes to the viral syndrome, which includes fever, runny nose (coryza), body aches, and fatigue. However, influenza can be more severe for individuals in high-risk groups, including those with chronic lung diseases, cardiovascular conditions, and pregnant women. In these populations, complications such as primary viral pneumonia, secondary bacterial pneumonia, hemorrhagic bronchitis, and death may occur. These severe complications can manifest rapidly, often within 48 hours of the onset of symptoms.

The influenza virus replicates in both the upper and lower respiratory tract. After initial exposure, replication begins almost immediately, with viral load peaking around 48 hours later. This rapid replication contributes to the acute nature of the disease.

Two key proteins on the surface of the influenza virus, hemagglutinin (H) and neuraminidase (N), are crucial for the virus’s ability to infect and spread. Hemagglutinin binds to epithelial cells in the respiratory tract, allowing the virus to enter and infect the cells. Neuraminidase cleaves the bond between the virus and infected host cells, facilitating the release and spread of new viral particles (virions) to other cells. The combination of these proteins defines the subtype of influenza viruses, such as H1N1 or H3N2.

An important characteristic of the influenza A virus is its genetic variability. The virus is genetically labile, meaning it has a high mutation rate, which leads to significant changes in its antigenic and functional proteins. This constant genetic shift allows the virus to evade immune detection, contributing to its ability to cause recurrent infections and complicating vaccine development.

RISK FACTORS OF INFLUENZA

The likelihood of contracting the flu or experiencing its complications can be influenced by several factors:

• Age – Children under 2 years and adults over 65 years are more likely to suffer severe outcomes from seasonal influenza.
• Living or Workplace Conditions – Individuals in crowded settings, such as nursing homes or military barracks, have an increased risk.
• Hospitalized patients are particularly vulnerable to influenza.
• Weakened Immune System – Conditions or treatments that weaken the immune system increase susceptibility, such as:
  • HIV/AIDSBlood cancers (e.g., leukaemia)Organ transplants requiring anti-rejection medicationLong-term steroid use
  • Cancer therapies
• Chronic Conditions – Pre-existing chronic illnesses heighten the risk of complications, including:
  • Asthma or other lung diseases
  • Diabetes
  • Heart disease or stroke
  • Neurological disorders
  • Metabolic disorders
  • Chronic kidney disease
  • Liver disease
• Race and Ethnicity – American Indian and Alaska Native populations may face a higher risk of flu-related complications.
• Aspirin Use Before Age 19 – Long-term aspirin therapy in individuals under 19 years increases the risk of Reye’s syndrome if they contract influenza.
• Pregnancy – Pregnant women are at greater risk, particularly during the second and third trimesters. This increased vulnerability persists for up to two weeks postpartum.
• Obesity – A high body mass index (BMI) is associated with a greater likelihood of severe flu complications.

Prognosis

Influenza is generally favourable for individuals without underlying health conditions, with most recovering fully within one to two weeks. However, some may experience lingering fatigue and weakness for several weeks. Infants and elderly individuals face the highest risk of severe outcomes, including influenza-related mortality, particularly due to complications like pneumonia. In high-risk groups, timely antiviral treatment can significantly reduce the chances of severe respiratory illnesses and hospitalizations. Although the case fatality rate remains below 1%, the widespread nature of influenza leads to a considerable number of deaths annually. Adults over 65 years old bear the greatest burden, accounting for more than 80% of fatalities and experiencing the highest hospitalization and mortality rates during seasonal flu epidemics.

Conclusion:

In conclusion, influenza is a widespread and highly contagious viral infection that poses significant health risks, particularly during seasonal epidemics. While most individuals recover with proper care, including rest, hydration, and symptom management, high-risk groups such as infants, the elderly, and those with underlying health conditions face a greater likelihood of severe complications. Preventative measures, especially annual flu vaccinations, remain the most effective defence against the disease. Prompt medical intervention, including antiviral treatments, can help mitigate the severity and reduce the risk of complications, emphasizing the importance of early detection and proactive care.

If you’re experiencing symptoms of influenza, visit our clinic for a comprehensive evaluation and personalized care plan. Our team of experienced professionals is dedicated to helping you recover quickly and safely by offering expert symptom management, tailored treatment strategies, and preventive care to support your overall health.