At a Glance
Influenza is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs, often leading to sudden fever, muscle aches, and fatigue throughout the body.
This widespread viral infection occurs globally in annual outbreaks and affects people of all ages, though young children and adults over age 65 are most frequently impacted by severe cases.
Influenza is an acute condition that typically resolves on its own within a few weeks but is treatable with antiviral medications if caught early.
Most generally healthy individuals recover fully within one to two weeks without long-term problems, although the condition can cause significant temporary disruption to quality of life.
How It Affects You
Influenza is a contagious viral infection that primarily targets the respiratory system but triggers a systemic immune response affecting the entire body. The virus attaches to cells in the nose, throat, and lungs, causing inflammation and respiratory distress, while the body's defense mechanisms produce widespread symptoms like fever and aching muscles. Significant effects on the body include:
- Systemic inflammation leading to high fever, chills, severe fatigue, and body-wide muscle aches.
- Respiratory obstruction and irritation causing persistent coughing, sore throat, and nasal congestion.
- Potential for secondary bacterial infections, such as pneumonia, which can severely compromise lung function.
Causes and Risk Factors
Underlying Causes and Transmission
Influenza is caused by influenza viruses, primarily types A and B, which circulate seasonally. The virus spreads mainly through tiny droplets made when people with the flu cough, sneeze, or talk. These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs. Less often, a person might get the flu by touching a surface or object that has the flu virus on it and then touching their own mouth, nose, or possibly their eyes. The virus invades the cells of the respiratory tract, hijacking the cells' machinery to replicate and spread, which triggers the immune system's inflammatory response.
Risk Factors
Anyone can get the flu, but certain factors increase the risk of developing the illness or experiencing severe complications. Age is a significant factor, with children under 5 years old and adults 65 years and older being more vulnerable. Living or working in crowded facilities, such as nursing homes or military barracks, increases exposure risk. People with weakened immune systems due to disease or medication, as well as those with chronic medical conditions like asthma, diabetes, or heart disease, are at higher risk. Pregnancy and obesity are also known factors that can increase the severity of the illness.
Primary Prevention
The most effective way to prevent influenza is by getting an annual flu vaccine. The vaccine causes antibodies to develop in the body about two weeks after vaccination, providing protection against infection with the viruses that are used to make the vaccine. Because flu viruses evolve quickly, the vaccine is updated each year to match the circulating strains. Vaccination is recommended for nearly everyone aged 6 months and older.
Reducing Transmission
Everyday preventive actions can help stop the spread of germs. These include avoiding close contact with people who are sick, covering coughs and sneezes with a tissue or the elbow, and washing hands often with soap and water. If soap and water are not available, an alcohol-based hand sanitizer is effective. Avoid touching the eyes, nose, and mouth, as germs spread this way. Cleaning and disinfecting frequently touched surfaces at home, work, or school, especially when someone is ill, also helps reduce the risk of infection.
Diagnosis, Signs, and Symptoms
Common Signs and Symptoms
Influenza symptoms typically begin suddenly, unlike a common cold which often develops gradually. People who have the flu often feel feverish or experience chills, though not everyone with the flu will have a fever. Common respiratory symptoms include a cough, sore throat, and a runny or stuffy nose. Systemic symptoms are also prominent and include muscle or body aches, headaches, and profound fatigue or tiredness. Some people may experience vomiting and diarrhea, though this is more common in children than adults.
Diagnostic Methods
Clinicians often diagnose influenza based on the patient's symptoms and clinical judgment, especially during known outbreaks. To confirm the diagnosis, doctors may use diagnostic tests. The most common are Rapid Influenza Diagnostic Tests (RIDTs), which detect the parts of the virus that stimulate an immune response. These tests can provide results in approximately 10 to 15 minutes but may not always be accurate. More sensitive tests, called molecular assays (such as RT-PCR), detect the genetic material of the virus and are more accurate but may take longer to process. These involve swiping the inside of the nose or the back of the throat with a swab.
Differential Diagnosis
Influenza is often confused with other respiratory illnesses due to overlapping symptoms. It is frequently distinguished from the common cold, which is generally milder and less likely to cause fever or body aches. It must also be differentiated from COVID-19, which shares many symptoms but may cause a loss of taste or smell and has a longer incubation period. Bacterial pneumonia, strep throat, and viral gastroenteritis are other conditions that healthcare providers may consider when evaluating a patient with flu-like symptoms.
Treatment and Management
Medical Treatment
For many people, treating influenza focuses on managing symptoms while the immune system fights the virus. Prescription antiviral drugs can treat the illness if started shortly after symptoms begin, ideally within 48 hours. These medications can make the illness milder, shorten the time a patient is sick, and prevent serious complications. They work by blocking the virus from reproducing in the body. Over-the-counter medications such as pain relievers and fever reducers are often used to alleviate muscle aches and high temperatures. It is important not to give aspirin to children or teenagers recovering from viral infections due to the risk of Reye's syndrome.
Home Management and Self-Care
Rest and hydration are the cornerstones of home management. Staying at home allows the body to direct energy toward the immune response and prevents spreading the virus to others. Drinking plenty of water, broth, or electrolyte solutions prevents dehydration, which can be a risk due to fever and loss of appetite. Using a humidifier can help soothe a sore throat and clear nasal congestion. Smokers should avoid smoking, as it irritates the airways and can worsen symptoms.
When to See a Doctor
Most people recover on their own, but immediate medical attention is necessary if warning signs appear. For adults, these include difficulty breathing or shortness of breath, persistent pain or pressure in the chest or abdomen, persistent dizziness, confusion, inability to arouse, seizures, or severe muscle pain. In children, red flags include fast breathing or trouble breathing, bluish lips or face, ribs pulling in with each breath, chest pain, dehydration (no urine for 8 hours, dry mouth, no tears), or a fever above 104°F. Emergency care is also needed if fever or cough improve but then return or worsen, as this may indicate a secondary infection.
Severity and Prognosis
Disease Course and Duration
Influenza is typically an acute, self-limiting illness. The incubation period—the time from infection to the start of symptoms—is usually about two days, but can range from one to four days. Uncomplicated influenza usually resolves after three to seven days for the majority of symptoms, although cough and malaise can persist for more than two weeks. People are generally contagious from one day before symptoms start until five to seven days after becoming sick.
Severity and Complications
While often mild to moderate, influenza can be severe and unpredictable. Complications can range from moderate issues like sinus and ear infections to serious, life-threatening conditions. Pneumonia is the most common serious complication and can result from the influenza virus itself or from a co-infection with bacteria. Other severe complications include inflammation of the heart (myocarditis), brain (encephalitis), or muscle tissues (myositis), and multi-organ failure. The condition can also trigger an extreme inflammatory response in the body known as sepsis. Individuals with pre-existing conditions like asthma or congestive heart failure may see a worsening of their chronic symptoms.
Prognosis
The prognosis for most healthy people who contract the flu is excellent, with full recovery expected. However, the prognosis depends heavily on the individual's age and underlying health status. Mortality risks are highest among the elderly, young infants, and those with significant comorbidities. Early treatment with antivirals improves the prognosis for high-risk patients by reducing the severity and duration of the illness. Vaccination significantly improves population-level outcomes by reducing the overall number of severe cases and hospitalizations.
Impact on Daily Life
Impact on Daily Activities
Influenza typically requires a complete halt to normal daily activities for several days. The sheer exhaustion and fever often make work, school, and household chores impossible during the acute phase. Isolation is necessary to protect others, which can lead to temporary social withdrawal. Parents often need to stay home to care for sick children or are unable to care for them if they fall ill themselves, creating logistical challenges. Even after the fever subsides, a lingering sense of fatigue can reduce productivity and energy levels for weeks, requiring a gradual return to full activity.
Questions to Ask Your Healthcare Provider
Being prepared for a medical appointment can help patients manage their condition effectively. Relevant questions to ask include:
- Do my symptoms suggest the flu or a different type of infection?
- Am I within the window where antiviral medications would be effective?
- What specific over-the-counter medications are safe for me to take with my other prescriptions?
- What are the specific warning signs that should prompt me to seek emergency care?
- How long should I stay away from work or school to avoid infecting others?
- Should other members of my household take preventive medication?
- Is it too late to get a flu shot for this season?
Common Questions and Answers
Q: Can the flu vaccine give me the flu?
A: No, the flu vaccine cannot cause the flu. The injected vaccine is made from an inactivated virus that is no longer infectious, or from a single gene from the flu virus (as opposed to the full virus) in order to produce an immune response without causing infection.
Q: Do I need antibiotics for the flu?
A: No, antibiotics are used to treat bacterial infections and do not work against viruses like influenza. Antibiotics may only be prescribed if you develop a secondary bacterial complication, such as bacterial pneumonia or an ear infection.
Q: Is the "stomach flu" the same as influenza?
A: No, the term "stomach flu" is a common nickname for viral gastroenteritis, which affects the intestines and causes vomiting and diarrhea. Influenza is a respiratory infection that centers on the nose, throat, and lungs, though it can sometimes cause stomach upset in children.
Q: How long is a person contagious?
A: A person may be able to pass on the flu to someone else before they know they are sick, as well as while they are sick. Healthy adults may be able to infect others beginning 1 day before symptoms develop and up to 5 to 7 days after becoming sick. Children and some people with weakened immune systems may pass the virus for longer than 7 days.
Q: Why do I need a flu shot every year?
A: Flu viruses evolve constantly, and the body's immune response from vaccination declines over time. An annual vaccine is needed for optimal protection because the vaccine formulation is updated each year to keep up with the changing flu viruses.