Flu - Symptoms

Influenza, commonly known as the flu, is an acute respiratory infectious disease caused by the influenza virus. Its symptoms usually appear suddenly and can vary widely. Patients may start experiencing symptoms 1 to 4 days after exposure to the virus. The initial signs may resemble those of a common cold but can quickly worsen. Understanding the progression of symptoms helps in early diagnosis and appropriate treatment.

Typical symptoms include fever, muscle aches, and fatigue, but manifestations can differ among different age groups and health conditions. Children may exhibit more pronounced symptoms due to an underdeveloped immune system; elderly individuals or those with chronic illnesses may need special attention due to a higher risk of complications. This article will detail the classification of influenza symptoms, their development stages, and emergency management strategies.

Early Symptoms and Signs

The early symptoms of influenza usually appear within 1 to 4 days after infection and may be mistaken for a common cold. The most common early signs include:

• Mild fever (around 38°C)
• General muscle soreness, especially in the shoulders, neck, and back
• Headache or throbbing headache
• Persistent fatigue that does not improve even after rest

These symptoms can rapidly intensify within hours, giving patients the sensation of having "suddenly fallen ill."

Some patients may experience atypical early symptoms, such as:

• Gastrointestinal discomfort, such as nausea or abdominal discomfort
• Sore throat or mild cough
• Sensitivity to light or sound

It is noteworthy that these signs may vary depending on the virus strain, for example, H1N1 may be associated with more gastrointestinal symptoms.

Common Symptoms

The typical symptoms of influenza peak between 2 to 7 days after onset, mainly including:

• High fever (often exceeding 39°C) accompanied by chills
• Severe muscle pain, sometimes affecting daily activities
• Dry cough or productive cough with phlegm, which may gradually worsen
• Nasal congestion or clear nasal discharge

These symptoms usually last for 3 to 7 days, but systemic fatigue can persist for weeks after recovery.

Respiratory symptoms may include:

• Sore throat with redness and pain on swallowing
• Cough that may become wet with yellow-green secretions
• Conjunctival injection or eyelid swelling in some patients

In immunocompromised patients, more severe symptoms such as difficulty breathing or altered consciousness may be observed.

Disease Progression and Symptom Changes

The development of influenza symptoms generally occurs in three stages:

Acute phase (1-3 days)

Symptoms worsen rapidly during this stage, with fever possibly reaching 40°C, and muscle soreness described as feeling "hit by a truck." Respiratory symptoms intensify quickly, and dehydration may occur.

Peak period (3-5 days)

Body temperature may remain high, and cough may alternate between dry and wet. Some patients may experience autonomic symptoms such as dizziness and lightheadedness due to fever. In children, high fever may trigger convulsions.

Recovery phase (5-7 days)

Fever gradually subsides, but fatigue can last for several weeks. During this stage, attention should be paid to secondary infections, such as bacterial pneumonia, which may be triggered by persistent cough.

When to Seek Medical Attention

Immediate medical help should be sought if high-risk symptoms occur:

• Fever exceeding 39.5°C lasting more than three days
• Difficulty breathing or oxygen saturation below 95%
• Confusion, incoherent speech, or disorientation
• Chest pain, rapid heartbeat, or pallor of extremities

These symptoms may indicate severe complications such as pneumonia or myocarditis.

Special attention should be given to vulnerable groups:

Emergency guidelines for high-risk populations

Elderly over 65, pregnant women, and patients with chronic diseases should seek outpatient care immediately if they experience:

• Fever with shortness of breath (adults >24 breaths/min)
• Petechiae or unexplained bruising
• Significant decrease in urine output within 24 hours
• Fever persists despite antipyretic medication

These signs may reflect viral invasion of organ systems.

Key indicators for emergency:

Immediate hospitalization indicators

If "three redness symptoms" (flushed face, red eyes, sore throat) or "three pain symptoms" (headache, muscle pain, orbital pain) appear, it may suggest severe influenza. Additionally, children experiencing seizures, refusing to eat, or showing skin rashes should be treated as emergencies.

 

Frequently Asked Questions

How to distinguish between influenza and a common cold?

Influenza often presents with high fever (above 39°C), whole-body aches, and severe fatigue, with rapid onset and more intense symptoms; common colds mainly feature nasal congestion, runny nose, lower fever, and milder systemic discomfort. If symptoms such as chest pain or rapid breathing occur, immediate medical attention is necessary.

Why can I still get infected after receiving the flu vaccine?

The effectiveness of the flu vaccine depends on how well the circulating virus strains match the vaccine strains. If the prevalent virus differs significantly from the vaccine, protection diminishes. Additionally, it takes about two weeks after vaccination to develop sufficient antibodies, so exposure to the virus before this period can still lead to infection. Therefore, continued precautions like frequent handwashing are recommended even after vaccination.

Is it normal for body temperature to fluctuate after taking antipyretics?

This is normal. Antipyretics temporarily suppress fever mechanisms but do not eliminate the virus. If the temperature rises again after the medication wears off, ensure adequate hydration and rest. If high fever persists for more than three days or consciousness becomes unclear, seek medical evaluation immediately.

Who are at high risk for influenza-related pneumonia?

Individuals over 65, those with chronic cardiovascular or respiratory diseases, immunocompromised persons, and obese individuals (BMI ≥30) are high-risk groups. These populations have higher rates of pneumonia following influenza infection. Symptoms such as coughing up blood or difficulty breathing warrant prompt chest X-ray and blood tests.

How long should I isolate after influenza infection before returning to normal social activities?

Generally, isolation should continue until the fever has been consistently reduced for more than 24 hours without the use of medication. It is recommended to isolate for about 7-10 days after symptom resolution to prevent transmission via respiratory droplets, especially avoiding contact with pregnant women, young children, and chronic disease patients.