You felt that fever that came out of nowhere, body aches that seemed impossible to bear, a pounding headache, and a weakness that kept you from getting out of bed. You thought: “it’s just the flu.”
But the flu is never “just” anything. It is the infectious disease that kills the most people worldwide every year — and one of the most misunderstood by the general public.
This article draws on data from the New England Journal of Medicine, the World Health Organization, the CDC, the UK Health Security Agency (UKHSA), the NHS, and the European Centre for Disease Prevention and Control (ECDC) to clarify once and for all what the flu is, how it acts in the body, who faces the greatest risk, and what science recommends for treatment and prevention.
Flu and cold: a confusion that can be costly
The first and most important thing to understand is that flu and the common cold are completely different diseases — caused by different viruses, with different levels of severity and very different consequences.
Both are respiratory infections caused by viruses. However, flu is caused primarily by the Influenza A and B viruses, while the common cold is caused by other respiratory viruses such as rhinovirus, respiratory syncytial virus (RSV), and parainfluenza virus.
| Feature | Flu (Influenza) | Common Cold |
|---|---|---|
| Onset | Sudden, abrupt | Gradual |
| Fever | High (100–104°F), common | Rare or low-grade |
| Body aches | Intense, widespread | Mild or absent |
| Fatigue | Intense, can last weeks | Mild |
| Cough | Dry, intense | Mild |
| Runny nose | Moderate | Intense |
| Complications | Possible and serious | Rare |
| Vaccine available | Yes | No |
According to the MSD Manuals, a global medical reference: “Influenza refers to the disease caused by influenza viruses, but the term is commonly used incorrectly to refer to similar illnesses caused by other respiratory viral pathogens.”
What is the Influenza virus and how does it act in the body
The Influenza virus belongs to the Orthomyxoviridae family and is divided into three main types: A, B, and C. Types A and B are responsible for the seasonal epidemics that occur every year. Type A is the most dangerous — it is the one that causes pandemics.
When the virus enters the body — usually through the upper airways — it attaches to cells of the respiratory epithelium and begins to replicate rapidly. The immune system detects the invasion and launches an intense inflammatory response. It is this immune response — not the virus directly — that causes most symptoms: fever, body aches, and fatigue.
The incubation period for flu is one to four days, with an average of two days. Transmission from infected individuals occurs one to two days before symptoms begin. The peak of viral shedding occurs between 24 and 72 hours after disease onset — meaning you can spread the flu before you even know you are sick.
Symptoms: what to expect and when to worry
A prospective national study published in 2025, tracking adults during the 2024/25 flu season, mapped the most frequent symptoms in the first week of infection:
- Cough — present in 75.2% of cases
- Stuffy or runny nose — 65% of cases
- Intense fatigue — 39% of cases
- Post-exertional malaise — 31% of cases
Fever is the most characteristic sign, especially in children. Most adults also experience muscle pain (myalgia), chills, general malaise, and headache. The typical flu illness is self-limiting — the body fights and eliminates the virus in 5 to 7 days, with cough potentially persisting for up to two weeks.
⚠️ When to seek urgent medical attention
- Difficulty breathing or shortness of breath
- Persistent pain or pressure in the chest
- Confusion or altered mental status
- Very high fever that does not respond to fever-reducing medication
- Bluish lips or fingernails
- Worsening symptoms after initial improvement
The 2025–2026 season: what changed and what is worrying experts
The 2025–2026 flu season introduced a new variant that put virologists on global alert: the Influenza A(H3N2) subclade K, also known as J.2.4.1.
According to an article published in JAMA in January 2026, signed by Dr. Frederick G. Hayden of Johns Hopkins Bloomberg School of Public Health: this variant, first detected in June 2025 in New York, rapidly became the dominant strain in multiple countries — the United States, United Kingdom, Australia, and parts of Africa and Asia. Influenza A(H3N2) is known for its high rate of evolution and tendency to cause severe epidemics, particularly in older adults.
In the United States, the season was particularly severe. An analysis published in JAMA described the 2024–2025 season as marked by “significant morbidity and mortality across all age groups, despite moderate vaccine effectiveness.” Early estimates of 2025–2026 flu vaccine effectiveness in England showed 70–75% effectiveness against flu-related hospitalization in children and 30–40% in adults, according to CDC data.
The European picture: what the ECDC is monitoring
In Europe, the leading authority on infectious disease monitoring is the ECDC — European Centre for Disease Prevention and Control — which publishes weekly reports on virus circulation across all EU and EEA countries.
According to the ECDC, in a typical season, influenza causes:
- Up to 50 million symptomatic cases per year in Europe
- Between 15,000 and 70,000 deaths annually in the EU
- Up to 20% of the European population contracts influenza each season
- Greatest impact on older adults and people with chronic conditions
- In long-term care facilities, outbreaks have high mortality rates
The 2025–2026 season was particularly concerning in Europe. According to an ECDC report published in November 2025, influenza activity in the region began three to four weeks earlier than in the two previous seasons. The H3N2 subclade K variant represented 47% of all A(H3N2) sequences deposited on the global GISAID database from 19 EU/EEA countries between May and November 2025.
The EuroMOMO mortality monitoring system recorded excess deaths above expected levels between week 51 of 2024 and week 9 of 2025, primarily affecting adults aged 45 and over, with impact increasing with age.
Bird flu H5N1: the threat the WHO is monitoring
Beyond seasonal flu, experts are watching a more serious threat: the avian Influenza A(H5N1) virus and its variants, which continue to spread among animals and cause sporadic human infections.
In February 2026, the World Health Organization announced new recommendations for the composition of vaccines for the Northern Hemisphere 2026–2027 season, following a four-day consultation examining global surveillance data. Since September 2025, 25 human infections with zoonotic influenza have been reported to the WHO from six countries. Most cases occurred in people exposed to infected animals. No human-to-human transmission has been reported to date.
Who faces the greatest risk of serious complications
For most healthy people, flu is a self-limiting illness. But for certain groups, it can progress to serious complications such as pneumonia, respiratory failure, and death.
According to the CDC and ACIP, those at highest risk include:
- Adults aged 65 and older — the group with the highest rates of flu-related hospitalization and death
- Pregnant women — at any stage of pregnancy
- Children under 5 years old — especially under 2, and particularly under 6 months
- People with chronic medical conditions — asthma, COPD, heart disease, diabetes, kidney disease, liver disease, and neurological conditions
- Immunocompromised individuals — including those on chemotherapy, corticosteroids, or living with HIV/AIDS
- People with obesity — especially those with a BMI of 40 or higher
- Residents of nursing homes and long-term care facilities
Treatment: what works and what does not
For most people, flu treatment is symptomatic — aimed at relieving discomfort while the immune system does its work:
- Rest — essential for recovery
- Hydration — water, warm teas, clear fluids
- Fever reducers and pain relievers — acetaminophen (paracetamol) or ibuprofen for fever and muscle aches
- Decongestants — for nasal congestion relief
⚠️ Important: aspirin should not be given to children or teenagers with flu — there is a risk of Reye’s syndrome, a serious condition affecting the liver and brain.
Antivirals: when are they indicated?
Antivirals are medications that act directly against the Influenza virus — and make a difference when used early. According to the MSD Manuals: “Antiviral medications reduce the duration of fever, severity of symptoms, and time to return to normal activities when administered within 1 to 2 days of symptom onset.”
The main antiviral available is oseltamivir (Tamiflu), which inhibits the viral neuraminidase. Others include zanamivir, peramivir, and baloxavir marboxil.
In the US, the CDC recommends antiviral treatment for all hospitalized patients and all high-risk patients with suspected or confirmed flu, regardless of time since symptom onset. For lower-risk outpatients, antivirals should ideally be started within 48 hours of symptom onset. In the UK, antivirals are similarly available through the NHS for high-risk groups — contact your GP as soon as symptoms begin.
The vaccine: what science says in 2025–2026
Why does it need to be taken every year?
The Influenza virus constantly mutates — what scientists call antigenic drift. This means the strains circulating in one year may differ from those of the previous year. The vaccine composition is updated annually based on global surveillance coordinated by the WHO. Previous years’ vaccines do not guarantee adequate protection in the current year.
Recommendations in the United States
According to the CDC’s Advisory Committee on Immunization Practices (ACIP), published in the Morbidity and Mortality Weekly Report (MMWR) for the 2025–26 season:
- Annual flu vaccination is recommended for everyone aged 6 months and older without a contraindication
- Vaccination should ideally be offered in September or October, before flu season peaks
- Adults aged 65 and older should receive high-dose, adjuvanted, or recombinant vaccines, which provide greater protection in this age group
- For most adults over 65 and pregnant women in the first or second trimester, vaccination during July and August should be avoided unless later vaccination may not be possible
- Children aged 6 months through 8 years receiving the vaccine for the first time need two doses, at least 4 weeks apart
- For 2025–26, the ACIP recommends only single-dose formulations free of thimerosal as a preservative
A new development for 2025–26: the FDA approved FluMist (nasal spray live attenuated vaccine) for self-administration or caregiver administration — a significant step in improving vaccine accessibility. FluMist is approved for individuals aged 2 through 49 years who are not pregnant and do not have certain health conditions.
In the US, flu vaccines are available at doctors’ offices, pharmacies, community health clinics, and many workplaces. Most insurance plans, including Medicare and Medicaid, cover flu vaccination at no cost. Those without insurance can find free or low-cost options through federally qualified health centers or programs like VaccineFinder.org.
Recommendations in the United Kingdom
In the UK, the NHS flu programme — guided by the Joint Committee on Vaccination and Immunisation (JCVI) — offers free flu vaccination to several groups. According to the UK Health Security Agency (UKHSA) guidance for 2025–2026:
- Everyone aged 65 and over
- Pregnant women — at any stage of pregnancy
- Children aged 2 and 3 years — via nasal spray at GP practices
- School-aged children (Reception to Year 11) — via nasal spray at school
- People aged 18–64 with clinical risk conditions — including chronic respiratory disease (asthma, COPD), heart disease, diabetes, kidney disease, liver disease, neurological conditions, and immunosuppression
- Carers — those receiving carer’s allowance or who are the main carer for an elderly or disabled person
- People living with immunocompromised individuals
- Frontline health and social care workers
The NHS recommends getting the flu jab in October or early November for optimal winter protection, as protection can wane over time. Adults receive the vaccine as an injection; children typically receive a nasal spray, which is quick and painless. The adult programme typically begins from 1 October each year.
As the UKHSA emphasizes: “The annual flu programme saves thousands of lives every year, and reduces GP consultations, hospital admissions and pressure on A&E.”
Does the vaccine give you the flu?
No. Injected flu vaccines used in both the US and UK do not contain live flu viruses — they cannot give you flu. What some people feel after vaccination (soreness at the injection site, low-grade fever, mild fatigue) is a normal immune response, not flu itself. These side effects are mild and typically resolve within 1 to 2 days.
Prevention beyond the vaccine
Vaccination is the most effective measure, but not the only one. Other proven strategies include:
- Frequent handwashing — with soap and water for at least 20 seconds, or use of alcohol-based hand sanitizer
- Respiratory etiquette — covering nose and mouth when coughing or sneezing, ideally with the elbow
- Avoiding crowded spaces — especially during peak flu season
- Keeping spaces well-ventilated — the virus spreads mainly through respiratory droplets in enclosed spaces
- Staying home when sick — transmission is highest in the first days of illness
What science is still investigating
Two of the most promising developments in 2026:
mRNA flu vaccines: just like the mRNA vaccines for Covid-19, researchers are developing mRNA vaccines for influenza that could be updated far more quickly than traditional vaccines — a huge advantage given the speed of viral mutation. The New England Journal of Medicine published early results in 2026 showing promising efficacy and safety data.
Pandemic risk monitoring: the WHO and CDC are intensifying surveillance of zoonotic variants — especially H5N1 and H9N2 viruses circulating in animals — that have the potential to adapt for efficient human-to-human transmission. This type of monitoring is what gave the world early warning during the Covid-19 pandemic.
The information in this article is based on publications from the New England Journal of Medicine (NEJM), JAMA, MSD Manuals, the World Health Organization (WHO), the CDC and ACIP, the UK Health Security Agency (UKHSA), the NHS, and the European Centre for Disease Prevention and Control (ECDC). This content is for informational and educational purposes only and does not replace medical consultation.