Influenza — commonly called the flu and historically known as the grippe — is an infectious disease that affects humans and many animal species, including numerous mammals and birds. Caused by several kinds of viruses, influenza can recur throughout a person’s life because the viruses change over time and different strains circulate in the population. The term influenza comes from Italian, meaning “influence.”
Cause and major virus types
Influenza is produced by viruses that belong to the family Orthomyxoviridae; these are RNA viruses. Among the types that infect people, influenza A and B are responsible for the seasonal outbreaks that occur each year. Influenza A viruses are further subdivided by surface proteins (hemagglutinin and neuraminidase) and can infect multiple species, while influenza B primarily circulates among humans. Influenza C usually causes milder illness and is less often associated with widespread outbreaks.
Symptoms and transmission
Typical features of influenza appear suddenly and can include:
- fever or feeling feverish/chills
- cough and sore throat
- muscle or body aches, headache
- fatigue and weakness
- some people also experience nasal congestion, vomiting, or diarrhea (more common in children)
The virus spreads mainly via respiratory droplets produced when an infected person coughs, sneezes, or talks, and by touching surfaces contaminated with the virus followed by touching the face. Seasonal patterns and community immunity influence how widely the virus spreads.
Prevention and treatment
Vaccination is the principal public health tool for reducing influenza illness and its complications; seasonal vaccines are updated regularly to match circulating strains. Other preventive measures include hand hygiene, respiratory etiquette, and staying home when sick to limit transmission.
When illness occurs, care focuses on relieving symptoms and preventing complications. Antiviral medications may be prescribed for some patients; these are most effective when started early in the course of infection. Supportive measures include rest, fluids, and treatment of fever and pain as appropriate.
Complications and groups at higher risk
While many people recover without medical intervention, influenza can lead to serious complications such as pneumonia, worsening of chronic medical conditions (for example, asthma or heart disease), and, in severe cases, hospitalization or death. Those at higher risk of complications include older adults, very young children, pregnant people, and individuals with weakened immune systems or certain chronic illnesses.
Public health and history
Large influenza epidemics and occasional pandemics have had major health and social impacts. Public health responses combine vaccination campaigns, surveillance to detect new strains, clinical guidance on treatment, and non-pharmaceutical measures to limit spread. Because influenza viruses evolve rapidly, ongoing monitoring and periodic vaccine reformulation are necessary to maintain protection at the population level.