Influenza: Symptoms, Types, Causes and Treatment

Influenza, commonly known as the flu, is much more than just a seasonal nuisance. It’s a complex infectious disease that sweeps across the globe every year, causing significant illness and even death, especially among vulnerable populations. Understanding its symptoms, the different types of influenza viruses, what causes the disease, and how it can be treated or prevented is key to reducing its impact. In this article, we’ll explore the core features of influenza, guided by the latest scientific evidence.

Symptoms of Influenza

When influenza strikes, it can range from a mild inconvenience to a severe, sometimes life-threatening illness. Recognizing its symptoms early can help with faster diagnosis and management, and also distinguish it from other respiratory infections like the common cold.

Common	Less Common	Severity Range	Sources
Fever	GI issues	Mild to severe	1,2,3,4,5
Cough	Watery eyes	Asymptomatic to fulminant	1,2,3,4,7
Fatigue	Sinus pain	Aches, malaise	1,2,3,4,6,9
Headache	Sore throat	Short or long	1,2,3,4,7

Table 1: Key Symptoms of Influenza

Typical Symptom Profile

The classic symptoms of influenza often begin abruptly. People typically feel "hit by a truck," with rapid onset of:

• Fever: Usually high and persistent
• Cough: Often dry and can linger
• Fatigue: Marked tiredness and weakness
• Muscle aches: Generalized body aches and pains
• Headache: Prominent and often severe
• Sore throat and nasal congestion: Common but not always present
• Chills, sweating, and general discomfort: Frequently reported

These symptoms are more intense than those seen with the common cold, and can incapacitate even healthy individuals for several days 1,2,3,4.

Symptom Variation and Additional Features

• Gastrointestinal symptoms (like nausea, vomiting, or diarrhea) occur more often in children than adults and are most frequently seen with certain influenza A strains, such as H1N1pdm09, but are generally less common than respiratory symptoms 5.
• Notably, symptom severity and combinations can vary by age, immune status, and influenza virus type, but overall, the clinical presentation is very similar between influenza A and B infections 9.
• Some individuals may remain asymptomatic or only experience mild symptoms, while others can progress to severe or even fulminant illness, especially if other health problems are present 7.

Symptom Mechanisms

Many of the hallmark symptoms—like fever, malaise, headache, and muscle aches—are due to the body’s immune response to the infection, particularly the release of cytokines 2. Respiratory symptoms are primarily a result of the virus infecting the respiratory epithelium.

Types of Influenza

Influenza viruses aren’t all the same, and understanding their differences is critical for diagnosis, treatment, and prevention strategies.

Type	Hosts	Severity	Pandemic Potential	Sources
Influenza A	Humans, birds, some mammals	Mild to severe	Yes	6,7,8
Influenza B	Humans	Moderate	No	6,7,8
Influenza C	Humans (rare)	Usually mild	No	7,8

Table 2: Types of Influenza Viruses

Influenza A

• Most common and diverse.
• Infects humans, birds, and some mammals.
• Responsible for both seasonal epidemics and global pandemics.
• Further classified into subtypes based on surface proteins: hemagglutinin (H) and neuraminidase (N)—e.g., H1N1, H3N2.
• Only certain subtypes cause severe disease in humans 6,7,8.

Influenza B

• Humans are the primary host.
• Causes seasonal epidemics, similar in symptoms and severity to type A, but does not cause pandemics.
• Two main lineages: Victoria and Yamagata 6,7,9.

Influenza C

• Rare and usually causes mild illness (like mild respiratory infections).
• Not associated with epidemics or pandemics 7,8.

Key Points on Clinical Differences

• The clinical symptoms produced by different types and subtypes are generally very similar, making laboratory testing necessary for precise identification 9,10.
• Pandemic potential is unique to Influenza A, due to its ability to undergo antigenic shift—mixing its segmented genome with those from animal strains to create novel viruses 8.

Causes of Influenza

Influenza is caused by infection with influenza viruses, but the story doesn’t stop there. The interplay between virus characteristics, host factors, and environmental conditions shapes the disease’s spread and impact.

Cause	Description	Impact Level	Sources
Influenza viruses (A, B, C)	RNA viruses with high mutation rates	Global	6,7,8
Antigenic drift	Gradual changes in viral genes	Seasonal epidemics	6,8
Antigenic shift	Sudden genetic reassortment	Pandemics	8
Host response	Immune reaction, susceptibility	Individual	2,8

Table 3: Main Causes and Evolutionary Drivers of Influenza

The Viruses

• Influenza viruses are single-stranded, segmented RNA viruses. Their major surface proteins, hemagglutinin (HA) and neuraminidase (NA), are crucial for infection and immune recognition 8.
• Influenza A and B cause most human illness; influenza C is less important clinically 6,7.

Viral Evolution: Antigenic Drift and Shift

• Antigenic drift involves gradual, continuous mutations in the viral genome, especially in HA and NA, allowing the virus to evade existing immunity and cause seasonal epidemics 6,8.
• Antigenic shift is a more dramatic reassortment event, typically in influenza A, combining genes from different strains (often across species), leading to new subtypes capable of causing pandemics 8.

Host and Environmental Factors

• Host immunity: Pre-existing immunity, either from previous infections or vaccination, shapes who gets sick and how severely 6.
• Environmental factors: Close human contact, especially in winter, and airborne transmission facilitate the spread 7.
• Zoonotic reservoirs: Birds and some mammals serve as reservoirs for novel influenza A strains, with potential for cross-species transmission 6,7,8.

Complications and Co-Infections

• Bacterial superinfections: Influenza can damage respiratory tract defenses, making secondary bacterial pneumonia a major cause of severe disease and death, especially from pathogens like Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus 12,13.
• Systemic spread: While primarily a respiratory infection, the virus (especially avian strains) can cause systemic disease in rare cases 8.

Treatment of Influenza

Effective management of influenza involves a combination of symptom relief, antiviral medications, and preventive strategies. Treatment recommendations depend on disease severity, patient risk factors, and time since symptom onset.

Approach	Examples	Target Group	Sources
Antivirals	Oseltamivir, Zanamivir	Early in illness, high-risk, severe cases	6,14,17
Symptom relief	Analgesics, fluids, rest	All patients	2,14
Prevention	Annual vaccination	General population	6,14
Adjunctive therapies	(e.g., corticosteroids)	Not routinely recommended	15,16

Table 4: Influenza Treatment and Prevention Strategies

Antiviral Medications

• Neuraminidase inhibitors (oseltamivir, zanamivir) are the mainstay of antiviral therapy.
  • Most effective when started within 48 hours of symptom onset.
  • Recommended for severe cases, those at high risk for complications, or hospitalized patients.
  • Can reduce symptom duration and risk of complications 6,14.
• New antivirals are in development, targeting different stages of the virus life cycle and offering hope for improved outcomes, especially for resistant strains 17,18.

Symptomatic Treatment

• Supportive care includes rest, hydration, and medications to reduce fever and pain (e.g., acetaminophen, ibuprofen).
• Symptom management is the primary approach for otherwise healthy people with mild illness 2.

Prevention

• Annual vaccination is the cornerstone of influenza prevention.
  • Vaccines are updated yearly to match circulating strains.
  • Effectiveness varies, particularly if there is an antigenic mismatch, but vaccination remains the best preventive measure 6.
• Hygiene measures like hand washing and respiratory etiquette also help reduce transmission 14.

Adjunctive and Investigational Therapies

• Corticosteroids are sometimes used in severe cases, but current evidence suggests they may be associated with increased mortality and hospital-acquired infections; routine use is not recommended without more robust evidence 15,16.
• Antibiotics are only indicated for secondary bacterial infections, not for the viral infection itself 12,13.
• Emerging treatments such as monoclonal antibodies and antivirals with new mechanisms are under investigation 17,18.

Special Considerations

• Early treatment is crucial for high-risk groups (e.g., elderly, very young, pregnant women, or those with underlying health conditions) and for those with severe or rapidly progressive disease 14.
• Laboratory testing can aid in diagnosis, especially during periods when influenza is circulating in the community 14.

Conclusion

Influenza remains a formidable public health challenge, but our understanding and tools to fight it continue to evolve. Here’s a summary of what we’ve covered:

• Influenza causes a rapid onset of symptoms like fever, cough, fatigue, and muscle aches, with variation based on age and virus type 1,2,3,4,9.
• There are three types of influenza viruses—A, B, and C—with A and B responsible for most human illness and A being the main driver of pandemics 6,7,8,9.
• The disease is driven by highly mutable RNA viruses capable of evading immunity and, in the case of type A, causing pandemics through antigenic shift 6,8.
• Treatment relies on early administration of antivirals, supportive care, and prevention through vaccination. Adjunctive therapies like corticosteroids are not routinely recommended due to potential risks 6,14,15,16,17.

Staying informed about influenza—its symptoms, causes, and the latest advances in treatment—empowers both individuals and communities to reduce its impact and safeguard public health.