What is Influenza (Flu) & Its Types

Introduction

Influenza, commonly called the flu, is an infectious disease that can lead to mild to severe illness, sometimes resulting in hospitalisation or even death¹.  

Influenza is caused by highly contagious viruses and can lead to seasonal outbreaks worldwide¹. According to the National Centre for Disease Control, India recorded 76,030 cases and 2,172 deaths (2019 to April 2025) from Seasonal Influenza A, with significant outbreaks in 2019 and 2024².  

This article explores what influenza is, its types, its transmission, diagnosis, treatment, and prevention. 

What is Influenza?

Influenza is a common respiratory infection caused by influenza viruses that include types A, B, C, and D, however, types A and B are primarily responsible for seasonal epidemics in humans. These viruses infect both the upper and lower parts of the respiratory system and primarily spread through respiratory droplets when an infected person coughs, sneezes, or talks, but can also be transmitted via surfaces that have already been contaminated³.  

While healthy individuals usually recover within a few days, influenza can lead to serious complications (such as pneumonia and even death) in high-risk groups like young children, the elderly, pregnant women, and those with weakened immune systems^1,3.  

Note: People with the flu are usually contagious 1 day before symptoms appear and remain so for 5 to 7 days after. 

What is the Difference Between Influenza and Common Cold?

Influenza (flu) and the common cold are respiratory infections caused by different viruses, with the flu typically producing more severe symptoms than the common cold. Let’s see the difference: 

• Onset: Influenza virus symptoms usually come on suddenly, while cold symptoms develop gradually^4,5. 
• Fever: A high fever is common with the flu, but rare with a cold^3,4. 
• Fatigue: The flu often causes significant fatigue and weakness, while the fatigue from a cold is typically mild^4,5. 
• Aches: Flu causes more intense body aches and headaches, unlike the mild discomfort seen with a cold⁶. 
• Cough: Both can cause coughing, but flu-related coughs are usually more severe and persistent⁶. 
• Complications: The flu can lead to serious complications like pneumonia, especially in high-risk groups⁶. Colds rarely result in severe health issues. 
• Duration: Cold symptoms often resolve within a few days, while the influenza virus symptoms can last one to two weeks or longer^3,5. 

Types of Influenza

There are four types of influenza viruses that affect humans to varying degrees: 

1. Influenza A

This type of influenza causes pandemics and epidemics: 

• Infects: Humans, birds, pigs, and other animals⁷ 
• Subtypes: Classified based on two proteins, hemagglutinin (H) and neuraminidase (N), such as, H1N1 and H3N2^8,9 
• Severity: Certain strains can cause the most severe illness⁷ 

2. Influenza B

This type of influenza causes seasonal outbreaks:  

• Infects: Humans⁷
• Lineages: Two major lineages, Victoria and Yamagata⁸
• Severity: Generally milder than A but can still cause significant illness^7,9

3. Influenza C

This causes mild respiratory illness: 

• Infects: Humans (occasionally pigs)⁷
• No subtypes or lineages^7,8 
• Severity: Usually causes mild symptoms like a cold; rarely causes epidemics⁹

4. Influenza D

This does not infect humans (currently): 

• Infects: Cattle and pigs⁷
• No subtypes or lineages^7,8 
• Severity: Not known to cause illness in humans, but should be monitored for possible crossover⁹

Here’s a quick comparison table to help you understand the key differences between the four types of influenza viruses^3,7,9.

Types of Influenza 

How Influenza Spreads

You may be wondering is influenza infectious?  

Yes, influenza is highly infectious. The virus mainly spreads through respiratory droplets expelled when an infected person coughs, sneezes, or talks. These droplets can land in the mouths or noses of nearby people or be inhaled into their lungs. Additionally, the virus can be transmitted by touching surfaces or objects contaminated with it, and then touching the face (particularly the nose, mouth, or eyes)^3,5. Influenza can also spread through tiny airborne particles that linger longer and travel farther, especially indoors. 

Each type of influenza virus (A, B, C, and D) spreads differently: 

• Influenza A: Spreads quickly and extensively, especially during seasonal epidemics. It infects humans and animals and is responsible for major flu pandemics. Contagious from about 1 day before symptoms start to up to 7 days after^4,7. 
• Influenza B: Only affects humans (seals occasionally) and causes seasonal outbreaks, typically less severe than A. Contagious for a similar duration as Influenza A^4,7,9. 
• Influenza C: Causes mild respiratory illness and spreads more slowly. It’s less common and not linked to epidemics, with lower contagiousness than A or B^7,9. 
• Influenza D: Mainly infects cattle and is not known to cause illness in humans. It does not contribute to human flu outbreaks^7,9.

Risk Factors for Influenza

Certain groups of people are at an increased risk of developing severe influenza complications. Key high-risk groups include: 

• Infants and young children (especially under 5 years of age)¹⁰ 
• Elderly adults (above 65 years)⁹
• Pregnant women (during all trimesters and the postpartum period) 
• People with chronic medical conditions (like asthma, diabetes, etc.)^3,9 
• People with weakened immune systems (including those with HIV/AIDS, undergoing cancer chemotherapy, organ transplant recipients, etc.) 

Diagnosis and Tests for Influenza

The diagnosis of influenza usually starts with a clinical assessment based on observable symptoms such as fever, cough, body aches, and fatigue. For confirmation, your doctor may use various diagnostic methods to detect influenza viruses. These may include: 

• Rapid Molecular Assays: These tests detect influenza virus nucleic acids with high sensitivity and deliver results very rapidly¹². 
• RT-PCR and Other Molecular Tests: Considered the gold standard, these tests identify influenza viral RNA and can distinguish between influenza A and B strains, including subtypes¹².
• Rapid Influenza Diagnostic Tests (RIDTs): These antigen detection tests provide results quickly but have variable sensitivity (sometimes lower), although its specificity is usually high. While convenient, negative results may require confirmation with molecular assays during high influenza activity¹².
• Immunofluorescence Assays: Uses a fluorescent microscope to detect influenza A and B antigens¹².
• Viral Culture: Though not useful for immediate clinical decisions due to slow turnaround (1–10 days), viral culture is important for surveillance and strain characterisation¹². 

Note: Nasal swabs or suction samples from the nose are the best for accurate flu testing. 

Treatment and Management of Influenza

Influenza often resolves itself without medical intervention³. However, treatment options are important for those at higher risk of complications or severe illness. 

• Antiviral Medications: Antiviral drugs are the main treatment options for influenza types A and B. They may also help reduce complications, hospitalisations, and mortality in high-risk populations.  Antiviral medications work best when started within 48 hours of symptom onset, reducing illness severity and duration by about a day¹³. Common medications include¹⁴: 
  • Oseltamivir 
  • Zanamivir 
  • Peramivir 
  • Baloxavir marboxil

Note: You should only take this medication if your doctor has prescribed it for your condition.

• Symptomatic Care: Healthy individuals with mild flu symptoms should stay home, rest, and avoid contact with other members of the family (to minimise the risk of transmission)¹³. Fever and pain-relieving medications, when taken under medical supervision, can help relieve fever, headache, and body aches. Options include: 
  • Acetaminophen (paracetamol)  
  • Ibuprofen 
  • Aspirin 

Note: Do not give aspirin to children as it has been associated with the risk of Reye’s Syndrome¹⁵. 

If your fever has been gone for at least 24 hours without using fever-reducing medicine and you feel better, it’s a sign of recovery. Isolation should continue for at least 24 hours after the fever ends to help prevent the spreading of the flu¹³.

How to Prevent Influenza

Prevention strategies are essential to reduce flu transmission and protect public health: 

• Flu Vaccination: It is recommended for everyone aged 6 months and older, annually. Additionally, high-dose vaccines are preferred for adults 65 years and older, as the immune response tends to weaken with age. Pregnant women and other high-risk groups are also strongly recommended to get vaccinated. 
• Hand Hygiene: Frequent handwashing with soap and water (for at least 20 seconds) or using alcohol-based hand sanitisers reduces the spread of flu viruses.  
• Avoiding Close Contact with Sick Individuals: Limiting exposure to people who have flu-like symptoms helps prevent infection¹⁴. 
• Disinfecting Surfaces: Regular cleaning of commonly touched surfaces (doorknobs, phones, keyboards) decreases virus transmission¹⁵. 
• Wearing Masks: During flu outbreaks or in crowded settings, wearing masks (especially well fitted surgical or N95 marks) can help reduce respiratory droplet spread.  
• Stay Home When Sick: If flu symptoms develop, staying home minimizes the risk of spreading the virus to others¹³. 

Complications of Influenza

While many cases of influenza resolve within a week, the virus can sometimes lead to serious complications, especially in vulnerable populations. Common influenza complications include: 

• Pneumonia³ 
• Sinus infections⁴ 
• Ear infections⁴
• Worsening of chronic conditions (e.g., asthma, diabetes)⁴

Other rare complications occurring only in severe cases include:  

• Myositis (muscle inflammation)  
• Acute Respiratory Distress Syndrome (ARDS occurs especially in patients with compromised immunity or underlying lung disease) 
• Myocarditis (heart muscle inflammation)  
• Encephalitis (brain inflammation)¹⁶ 
• Organ failure (multiple organs)¹⁶

When to See a Doctor

Early medical care can help avoid serious complications. Seek medical attention immediately if you experience¹⁶:

• Symptoms that suddenly worsen, e.g., worsening cough, increased shortness of breath, confusion, or persistent vomiting 
• Difficulty breathing/shortness of breath 
• Persistent high fever (not reduced with medications) 
• Severe weakness or fatigue 
• Lips or face turning blue (indicating cyanosis and hypoxia)  
• Significantly decreased urine output or inability to urinate. 
• Chest pain or pressure (may indicate complications like pneumonia, myocarditis, or cardiac events) 
• Seizures (indicate severe complications like brain damage) 

Conclusion

The introduction of influenza as a major public health concern highlights the need for effective prevention and treatment strategies. It is a potentially serious illness that spreads easily and can lead to severe complications if not managed properly. Therefore, recognising symptoms early, seeking timely medical care, and taking preventive measures are key to reducing its impact. While early antiviral treatment (under medical advice) can help reduce the risk of severe outcomes, annual vaccination remains the most effective way to protect against seasonal flu.  

Frequently Asked Questions (FAQs)

References

1. World Health Organization. Influenza – India [Internet]. World Health Organization; [cited 2025 May 28]. Available from: https://www.who.int/india/health-topics/influenza-seasonal 
2. National Centre for Disease Control. Seasonal Influenza [Internet]. Ministry of Health and Family Welfare, Government of India; [cited 2025 May 28]. Available from: https://ncdc.mohfw.gov.in/seasonal-influenza-2/ 
3. Boktor SW, Hafner JW. Influenza. StatPearls [Internet]. StatPearls Publishing; 2025 Jan; Available from: https://www.ncbi.nlm.nih.gov/books/NBK459363/ 
4. Centers for Disease Control and Prevention. About influenza [Internet]. Centers for Disease Control and Prevention; [cited 2025 May 28]. Available from: https://www.cdc.gov/flu/about/index.html 
5. Centers for Disease Control and Prevention. About common cold [Internet]. Centers for Disease Control and Prevention; [cited 2025 May 28]. Available from: https://www.cdc.gov/common-cold/about/index.html 
6. National Institutes of Health. Cold, flu, or allergy? [Internet]. National Institutes of Health; 2014 Oct [cited 2025 May 28]. Available from: https://newsinhealth.nih.gov/2014/10/cold-flu-or-allergy 
7. Liang Y. Pathogenicity and virulence of influenza. Virulence. 2023 Dec;14(1):2223057. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC10283447/ 
8. Centers for Disease Control and Prevention. Types of influenza viruses [Internet]. Centers for Disease Control and Prevention; [cited 2025 May 28]. Available from: https://www.cdc.gov/flu/about/viruses-types.html 
9. World Health Organization. Influenza (seasonal) [Internet]. Geneva: World Health Organization; 2025 Feb 28 [cited 2025 May 28]. Available from: https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal) 
10. Huang QS, Bandaranayake D, Wood T, et al. Risk Factors and Attack Rates of Seasonal Influenza Infection: Results of the Southern Hemisphere Influenza and Vaccine Effectiveness Research and Surveillance (SHIVERS) Seroepidemiologic Cohort Study. J Infect Dis. 2019 Jan 9;219(3):347-357. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC9006182/ 
11. Bhalerao-Gandhi A, Chhabra P, Arya S, Simmerman JM. Influenza and pregnancy: a review of the literature from India. Infect Dis Obstet Gynecol. 2015;2015:867587. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC4355110/ 
12. Centers for Disease Control and Prevention. Overview of influenza testing methods [Internet]. Centers for Disease Control and Prevention; [cited 2025 May 28]. Available from: https://www.cdc.gov/flu/hcp/testing-methods/index.html 
13. Centers for Disease Control and Prevention. Treatment of Flu [Internet]. Centers for Disease Control and Prevention; [cited 2025 May 28]. Available from: https://www.cdc.gov/flu/treatment/index.html 
14. Santibañez S, Fiore AE, Merlin TL, Redd S. A primer on strategies for prevention and control of seasonal and pandemic influenza. Am J Public Health. 2009 Oct;99 Suppl 2(Suppl 2):S216-24. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC4504386/ 
15. Baek JH, Seo YB, Choi WS, Kee SY, Jeong HW, Lee HY, Eun BW, Choo EJ, Lee J, Kim SR, Kim YK, Song JY, Wie SH, Lee JS, Cheong HJ, Kim WJ; Transgovernmental Enterprise for Pandemic Influenza in Korea. Guideline on the prevention and control of seasonal influenza in healthcare setting. Korean J Intern Med. 2014 Mar;29(2):265-80. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC3957004/ 
16. Centers for Disease Control and Prevention. Signs and Symptoms of Flu [Internet]. Centers for Disease Control and Prevention; [cited 2025 May 28]. Available from: https://www.cdc.gov/flu/signs-symptoms/index.html 
17. Press Information Bureau, Government of India. Update on Seasonal Influenza. [Internet]. Ministry of Health and Family Welfare; [updated 2023 Mar 10; cited 2025 May 28]. Available from: https://pib.gov.in/PressReleasePage.aspx?PRID=1905602 
18. Centers for Disease Control and Prevention. About Influenza A in Animals [Internet]. Centers for Disease Control and Prevention; [cited 2025 May 28]. Available from: https://www.cdc.gov/flu-in-animals/about/index.html 

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