Influenza – Symptoms and Causes

What Is Influenza?

Influenza, also known as the flu, is a viral infection that affects the respiratory system. It targets the nose, throat, and lungs. This illness is different from stomach viruses that cause digestive problems like diarrhea and vomiting.

Most people recover from the flu without special treatment. However, in some cases, influenza can lead to serious complications that may be life-threatening.

Flu Signs and Symptoms

Flu viruses spread widely during specific times of the year. In North America, flu season typically occurs between October and May. While flu symptoms might seem similar to a cold, they usually appear more suddenly and feel more severe.

Common symptoms of flu include:

• Fever
• Cough
• Sore throat
• Runny or stuffy nose
• Headache
• Muscle aches
• Extreme fatigue
• Sweating and chills

Children with flu may show irritability or fussiness instead of being able to describe specific symptoms. They’re also more likely to experience:

• Ear pain
• Nausea
• Vomiting
• Diarrhea

Some people might also have eye pain, watery eyes, or sensitivity to light sensitivity when infected with the flu.

When Medical Care Is Needed

Most people can treat the flu at home without seeing a doctor. However, certain situations require medical attention.

People at high risk for complications should see a healthcare provider promptly when flu symptoms appear. Starting antiviral medication within 48 hours of symptom onset can reduce illness duration.

Seek emergency medical care for these warning signs in adults:

• Difficulty breathing
• Chest pain or pressure
• Persistent dizziness
• Confusion or inability to wake
• Signs of dehydration
• Seizures
• Worsening of existing health conditions
• Severe weakness or muscle pain

For children, emergency warning signs include:

• Rapid breathing or chest retractions
• Bluish or grayish skin color (especially on the lips or nail beds)
• Dry mouth, lack of tears, and reduced urination
• Symptoms that improve but then return with a worse fever or cough

Don’t wait to get help if these emergency symptoms develop—they can indicate serious complications that require immediate treatment.

Causes

Influenza results from viral infection. These viruses spread through airborne droplets when infected individuals cough, sneeze, or talk. You can become infected by directly breathing in these droplets or by touching contaminated surfaces and then touching your face.

The contagious period typically begins about one day before symptoms appear and continues for 5–7 days after symptoms start. Children and people with weakened immune systems may remain contagious for a longer period.

Influenza viruses constantly evolve, creating new strains. This evolution explains why people can get the flu multiple times throughout their lives.

When someone gets infected with influenza for the first time, their body develops some lasting protection against similar viral strains. However, this protection is limited because of how frequently the virus changes.

Risk Factors

Age Concerns

The flu can be especially dangerous for certain age groups. Children under 2 years old face higher risks of serious flu complications.

Similarly, adults over 65 are more vulnerable to severe flu illness and complications. These age groups often need extra protection during flu season.

Group Housing Situations

People living in crowded facilities like nursing homes have a higher chance of catching the flu. Close living quarters make it easier for flu viruses to spread quickly from person to person.

Immune System Problems

When your immune system isn’t working properly, you may get sick more easily and have worse symptoms from the flu.

Immune system weakness can be congenital, caused by illnesses, or a side effect of treatments and medications that suppress immune function.

Long-Term Health Conditions

Several ongoing health problems can make flu complications more likely:

• Lung conditions like asthma
• Diabetes
• Heart disease
• Nervous system disorders
• Previous stroke
• Kidney, liver, or blood diseases
• Metabolic disorders
• Airway problems

Racial and Ethnic Factors

In the US, some groups face higher risks of needing hospital care for flu, including:

• Native American and Alaska Native individuals
• Black individuals
• Latino populations

Long-term Aspirin Use

Young people taking aspirin for extended periods may develop Reye’s syndrome if they catch the flu. This rare but serious condition affects the brain and liver.

Pregnancy Status

Pregnant women face increased risk of flu complications, particularly during the second and third trimesters. The flu can also potentially affect the developing baby.

Weight Factors

People with a BMI of 40 or higher (severe obesity) are at a higher risk for serious complications from flu infections. This may be related to how excess weight affects immune function and breathing.

Complications

The flu often passes without long-term issues for young, healthy people. Though unpleasant, symptoms typically resolve within 1–2 weeks.

However, those in high-risk groups may develop serious complications after influenza infection. Secondary infections are common complications, including:

• Croup
• Sinus infections
• Ear infections
• Lung infections

The flu can sometimes affect other body systems, leading to:

• Heart muscle inflammation
• Heart lining infection
• Central nervous system infections

Additional serious complications may include:

• Acute respiratory distress syndrome
• Muscle damage (rhabdomyolysis)
• Muscle inflammation (myositis)
• Toxic shock syndrome

People with existing health conditions face greater risks, as the flu can worsen chronic illnesses like asthma and kidney disease.

Ways to Prevent the Flu

Getting Flu Vaccines

The CDC recommends yearly flu vaccinations for everyone 6 months of age and older, unless there are medical contraindications. While not perfect, the vaccine significantly reduces the risk of severe illness, particularly for high-risk groups.

Getting vaccinated helps in several ways:

• Lowers the risk of catching the flu
• Reduces chances of serious illness requiring hospitalization
• Decreases risk of flu-related death
• Protects newborns when given later in pregnancy

The 2024-2025 seasonal flu vaccines protect against three influenza viruses expected to be most common this season. Vaccines come in different forms:

Vaccine Type	Age Range	Notes
Shot	All eligible ages	Given in arm muscle for older children/adults, thigh for younger children
Jet injector	Adults under 65	Uses a high-pressure fluid stream instead of a needle
Nasal spray	Ages 2–49	Has several restrictions (see below)

The nasal spray isn’t recommended for:

• People with severe allergic reactions to previous flu vaccines
• Pregnant individuals
• Young people taking aspirin or salicylate medicines
• Those with weakened immune systems or their caregivers
• Children ages 2–4 with asthma or a history of recent wheezing
• People who recently took antiviral flu medicine
• Individuals with cerebrospinal fluid leaks

Special vaccines called high-dose or adjuvanted influenza vaccines may help reduce hospitalization risks. These are recommended for people over 65 and for those 18 and older who have received solid organ transplants and take immunosuppressive medications.

People with egg allergies can still get flu vaccines. Children between 6 months and 8 years might need two doses their first time, given at least four weeks apart. After that, they need only one dose yearly.

Consult your healthcare provider before getting vaccinated if you have had serious reactions to previous flu vaccines or have had Guillain-Barré syndrome. If you feel sick on vaccination day, ask if you should wait.

Stopping Infection Spread

Since flu vaccines aren’t 100% effective, you should take these extra steps to reduce infection spread:

1. Wash hands thoroughly with soap and water for at least 20 seconds. Use hand sanitizer with 60%+ alcohol when soap isn’t available.

2. Keep hands away from your face. This will prevent germ transfer, especially to your eyes, nose, and mouth.

3. Cover coughs and sneezes with a tissue or your elbow, then wash your hands.

4. Clean surfaces regularly. Pay special attention to frequently touched areas to prevent picking up the virus from surfaces.

5. Avoid crowds during peak flu season. The flu spreads easily in places like child care centers, schools, office buildings, auditoriums, and public transportation.

6. Ensure good air circulation in indoor spaces to help reduce airborne virus concentrations.
7. Stay home if you’re sick until you feel better and have been fever-free for at least 24 hours without medication. This helps protect others from infection.

Combining annual flu vaccination with these preventive habits provides the best defense against influenza and its potential complications, especially for vulnerable populations.