What Is the Influenza Virus?
The influenza virus is an acute, highly contagious viral infection caused by viruses belonging to the Orthomyxoviridae family. It primarily affects the respiratory tract, including the nose, throat, and lungs, and is responsible for seasonal flu epidemics every year as well as occasional global pandemics.
Historical Impact of Influenza
Although seasonal flu virus remains a major public health concern, far more lethal influenza strains have emerged periodically.
The most devastating outbreak occurred in 1918, known as the Spanish flu. Despite its name, early cases were reported in the United States and parts of Europe. This pandemic infected nearly one-third of the world’s population and caused an estimated 20–50 million deaths worldwide, including approximately 549,000 deaths in the United States.
This history highlights the pandemic potential of the influenza virus, especially when novel strains emerge.
Types of Influenza Virus (Genera)
The major human-infecting genera include:
- Alphainfluenzavirus → Influenza A
- Betainfluenzavirus → Influenza B
- Gammainfluenzavirus → Influenza C
- Deltainfluenzavirus → Influenza D (not known to significantly infect humans)
Classification
The influenza viruses are classified into four main types:
A. Influenza A
- Most common and severe.
- Infects humans, birds, pigs, horses, and other animals.
- Responsible for seasonal epidemics and all major pandemics.
- Subtyped based on:
- Hemagglutinin (H): H1–H18
- Neuraminidase (N): N1–N11
Example: H1N1, H3N2
B. Influenza B
- Infects almost exclusively humans.
- Causes seasonal outbreaks but no pandemics.
- Two lineages:
- Victoria and Yamagata.
C. Influenza C
- Causes mild respiratory illness.
- No epidemics or pandemics.
D. Influenza D
- Affects cattle and pigs.
- Not known to infect humans significantly.
How Influenza Spreads (Transmission)
The influenza virus spreads mainly through:
1. Respiratory Droplets
- Released during coughing, sneezing, or talking
2. Aerosols
- Tiny particles that remain suspended in air
3. Fomites
- Touching contaminated surfaces and then the mouth or nose
Incubation Period
- 1–4 days (average: 2 days)
Contagious Period
- From 1 day before symptoms to 5–7 days after symptom onset
Pathogenesis (How the Virus Causes Disease)
- Virus enters via nose, throat, or eyes.
- HA attaches to host cell receptors (sialic acid).
- Virus enters and releases RNA into the cell.
- Rapid replication in respiratory epithelial cells.
- Cell death → inflammation → symptoms (fever, cough, myalgia).
- Immune response causes many systemic symptoms.
Signs & Symptoms
The viruses that cause flu spread at high levels during certain times of the year in the Northern and Southern hemispheres. These are called flu seasons. In North America, flu season usually runs between October and May.
Symptoms of the flu such as a sore throat and a runny or stuffy nose are common. You may also get these symptoms with other illness such as a cold. But colds tend to start slowly, and the flu tends to come on quickly, within two or three days after you come in contact with the virus. And while a cold can be miserable, you usually feel much worse with the flu.
other common flu symptoms include
- High fever (up to 104°F)
- Sudden chills
- Sore throat
- Runny or blocked nose
- Dry cough
- Muscle aches (myalgia)
- Severe fatigue
- Headache
- Loss of appetite
Complications
- Viral pneumonia
- Secondary bacterial pneumonia (e.g., Staphylococcus aureus)
- Exacerbation of chronic diseases (asthma, COPD)
- Myocarditis, encephalitis (rare)
- Hospitalization and death in high-risk groups
When to see a doctor
Most people with influenza can recover at home with rest and supportive care and usually do not need to see a healthcare professional.
However, individuals who develop flu symptoms and are at higher risk for complications should contact a healthcare provider as soon as possible. Antiviral treatment started within 48 hours of symptom onset can shorten the duration of illness and reduce the risk of serious complications.
Seek immediate medical attention if emergency flu symptoms occur. In adults, warning signs include:
- Difficulty breathing or shortness of breath
- Chest pain or persistent pressure
- Ongoing dizziness or confusion
- Difficulty waking up or sudden mental changes
- Signs of dehydration
- Seizures
- Worsening of existing medical conditions
- Severe weakness or intense muscle pain
Laboratory Diagnosis
1. RT-PCR (gold standard)
- Detects viral RNA
- Identifies subtype (H1, H3, etc.)
2. Viral Culture
- Used for scientific surveillance
3. Rapid Antigen Tests
- Quick, but low sensitivity
4. Serology
- Mainly for research, not routine diagnosis
Treatment
A. Neuraminidase Inhibitors
- Oseltamivir
- Zanamivir
- Peramivir
Work best within 48 hours of symptom onset.
B. Endonuclease Inhibitor
- Baloxavir marboxil
- Stops viral replication early
- Useful in young adults & high-risk patients
C. Supportive Management
- Hydration
- Antipyretics
- Monitor high-risk groups for complications
Vaccines
Types
- Inactivated influenza vaccine (IIV)
- Live attenuated influenza vaccine (LAIV)
- Recombinant vaccine (FluBlok)
- mRNA influenza vaccines (in advanced trials as of 2025)
Vaccine Challenges
- Short lifespan of immunity
- Strain mismatch due to rapid viral evolution
- Need for annual reformulation
Whats difference between Antigenic Drift & Shift
Antigenic Drift
- Small point mutations in HA & NA
- Cause yearly epidemics
- Require annual vaccine updates
Antigenic Shift
- Major reassortment event between different strains
- Only occurs in Influenza A
- Leads to pandemics
Recent Scientific & Public-Health Updates on flu virus
Global Circulation & Surveillance (2025)
- As of late November 2025, the World Health Organization (WHO) reports that flu virus activity worldwide remains elevated. Influenza A viruses predominate in most regions. World Health Organization
- Among subtypes, in 2025 in many temperate and subtropical zones, the strains A(H1N1)pdm09 and A(H3N2) are the main viruses detected. World Health Organization+2CDC+2
- For the 2025–2026 influenza illness season in the Northern Hemisphere, WHO has recommended vaccine compositions based on circulating strains — including: an H1N1pdm09-like virus, an H3N2-like virus, and a B-lineage virus (B/Victoria lineage) for trivalent vaccines. World Health Organization+2World Health Organization+2
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