Influenza A typically causes a sudden onset of fever, cough, sore throat, body aches, and fatigue. Symptoms usually appear one to four days after exposure and follow a predictable pattern, with the worst hitting in the first three days and most people recovering within one to two weeks. Not everyone experiences every symptom, and some groups, particularly young children and older adults, can present differently.
Core Symptoms in Adults
The hallmark of influenza A is how fast it hits. Unlike a cold that builds gradually, the flu tends to arrive all at once. The most common symptoms include:
- Fever or chills (though not everyone develops a fever)
- Cough, usually dry at first
- Sore throat
- Runny or stuffy nose
- Muscle and body aches
- Headaches
- Fatigue, sometimes severe
Some adults also experience vomiting and diarrhea, though gastrointestinal symptoms are far more common in children. The muscle aches can be intense enough to make even lying in bed uncomfortable. That soreness happens because your immune system floods your bloodstream with signaling proteins called cytokines to fight the virus. Those same proteins trigger widespread inflammation, which is why your entire body can feel painful even though the virus is concentrated in your respiratory tract.
How Symptoms Progress Day by Day
The flu follows a fairly consistent arc. During the first three days, fever, headache, muscle pain, weakness, a dry cough, and sore throat dominate. This is typically the most miserable stretch, when fevers run highest and fatigue is at its worst.
Around day four, fever and body aches start to ease. In their place, the cough and sore throat become more prominent, sometimes with a hoarse voice and mild chest discomfort. Energy levels stay low. By about day eight, most symptoms are noticeably better, but a lingering cough and tiredness can stick around for one to two weeks beyond that, sometimes longer. That extended fatigue catches many people off guard; feeling wiped out well after other symptoms clear is normal with the flu.
Symptoms in Children
Children develop the same respiratory symptoms as adults, including fever, cough, sore throat, congestion, body aches, and fatigue. The key difference is that young children are more likely to have vomiting and diarrhea alongside those symptoms. Infants and toddlers who can’t describe how they feel may simply appear unusually irritable, lethargic, or unwilling to eat or drink. High fevers tend to be more common in children, and dehydration from a combination of fever, poor appetite, and vomiting can develop quickly.
Symptoms in Older Adults
Influenza A can look quite different in people over 65. Older adults, especially those who are frail or immunosuppressed, often do not develop a fever at all. Without that obvious signal, the flu can be mistaken for general fatigue or a mild cold.
In long-term care settings, the CDC notes that atypical presentations are common: confusion, behavioral changes, and loss of appetite may be the only signs. These mental status changes can be mistaken for other conditions, which is one reason flu outbreaks in nursing facilities can spread before they’re identified. If an older person suddenly becomes confused, unusually drowsy, or stops eating during flu season, influenza should be on the list of possible causes.
Influenza A vs. Influenza B
From a symptom standpoint, influenza A and B are nearly identical. Testing is the only reliable way to tell them apart. That said, influenza A tends to be associated with more severe symptoms and higher fevers, particularly during large outbreaks. Influenza B, meanwhile, shows up more often in children and may lean slightly more toward gastrointestinal symptoms like nausea and vomiting in younger age groups. These are tendencies rather than rules. Either type can cause serious illness in anyone.
Signs the Flu Has Become Something Worse
Most people recover from influenza A without complications. The concern is when the infection leads to something secondary, most commonly pneumonia. Pneumonia can develop when the flu weakens your respiratory defenses enough for bacteria to take hold, or when the virus itself spreads deeper into the lungs.
The pattern to watch for is a “second wave” of illness. You start to improve, then suddenly feel worse again. Specific warning signs that suggest pneumonia or another serious complication include:
- Difficulty breathing or shortness of breath, especially while sitting still
- Chest pain, particularly when coughing or breathing deeply
- A fever of 102°F (38.9°C) or higher that returns after initial improvement
- Coughing up yellow, green, or bloody mucus
- Confusion or difficulty thinking clearly
- Bluish tint to lips or nails, which signals low oxygen
In children, watch for fast or labored breathing, an inability to keep fluids down, extreme irritability, or a fever that keeps climbing despite treatment. Any of these signs warrant immediate medical attention, as bacterial pneumonia progresses quickly and requires different treatment than the flu itself.