Influenza
Influenza is a seasonal viral respiratory illness that ranges from mild to severe, with the CDC Yellow Book and ACIP maintaining an annually updated list of high-risk groups.
Symptoms
Sudden fever, chills, body aches, headache, fatigue, cough, and sore throat -- generally more abrupt and severe than a cold.
Causes and risk
Influenza A and B viruses spread by respiratory droplets. High-risk groups per the 2025-26 ACIP recommendations include adults 65+, pregnant people, young children, and those with chronic heart, lung, kidney, or immune conditions.
How it is evaluated
Molecular (NAAT/PCR) testing is the gold standard per 2024 IDSA/ASM guidance, though rapid antigen tests are common with known limited sensitivity. Many outpatients, including telehealth patients, can be diagnosed and treated clinically without testing.
Treatment
For higher-risk, severe, or hospitalized patients, the CDC recommends starting antiviral therapy (oseltamivir, baloxavir, zanamivir, or peramivir depending on age and eligibility) as early as possible, ideally within 48 hours of symptom onset, per current CDC and FDA labeling guidance.
When to seek care
Difficulty breathing, chest pain, persistent dizziness, confusion, severe vomiting, or symptoms that improve then return worse all warrant urgent evaluation -- these are standard CDC flu emergency warning signs.