Influenza – Diagnosis, Symptoms, and Treatment

Medically Reviewed by Dr. Po-Chang Hsu, MD, MS

Medically reviewed by Dr. Po-Chang Hsu, MD, MS

Medical Professional

Updated - January 4, 2021

Dr. Po-Chang Hsu is a medical doctor from Tufts University in Boston, Massachusetts, interested in pediatrics and neonatology.

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What is Influenza?

Influenza is a virus that causes a syndrome characterized by fever, headache, respiratory symptoms, body aches (myalgias), abdominal discomfort, nausea, and vomiting.  The virus is prevalent, especially during the fall and winter seasons, likely because large groups of people tend to gather indoors and transmit the infection.  Respiratory droplets typically spread influenza.

The condition is highly contagious and is a leading cause of primary care clinic and hospital visits.  Complications include bacterial pneumonia, respiratory failure, sepsis, shock, and death.  Influenza virus infection leads to a considerable amount of deaths every year. This is usually seen in young children and elderly adults with other comorbidities.

What Causes Influenza?

Influenza is caused by infection with the influenza virus type A or B.  There are subtypes of influenza A and B, such as the notorious influenza H1N1.  Patients typically acquire the virus via respiratory transmission by close sick contacts.

How Common is Influenza?

It is prevalent and tends to occur seasonally, usually during fall or winter.  It is a leading cause of primary care and hospital visits during the fall and winter months. Infections tend to occur in epidemics and sometimes pandemics.  This is likely a result of the fact that individuals tend to gather indoors during winter.

Death rates are higher in the elderly and infants during epidemics.  However, during pandemics, there also appears to be a high death rate in young adults age 20-40.  The 2009 pandemic was associated with increased death rates and morbidity in children and young adults.

Signs and Symptoms

The following symptoms typically characterize influenza and influenza-like syndromes:

  • Fever (Temp > 101 F)
  • Body aches (myalgias)
  • Upper respiratory symptoms – runny nose (rhinorrhea), nasal congestion, and sore throat (pharyngitis)
  • Gastrointestinal symptoms – nausea, vomiting, abdominal discomfort, and diarrhea

Symptoms usually occur quickly within a couple of days. The virus can be debilitating and frequently lead to work absences.

Patients with confusion, high persistent fever, vomiting and inadequate oral intake, difficulty breathing, cough, wheezing, or chest pain should be evaluated for influenza complications.  These may include:

  • Pneumonia
  • COPD or asthma exacerbation
  • Decompensated heart failure
  • Kidney (renal) failure
  • Sepsis & shock
  • Death

.  These conditions typically occur in the elderly with other comorbidities such as:

  • COPD
  • Heart failure
  • End-stage renal disease
  • Chronic liver disease
  • Immunosuppression – HIV/AIDS, cancer, chemotherapy


Influenza is suggested based on symptoms and physical examination findings.  This is particularly true if symptoms occur during fall or winter and the patient has known exposure to sick contacts.  It often occurs in epidemics in which large numbers of people develop the infection since it is easily transmittable and highly contagious.

The diagnosis is usually confirmed with laboratory tests.  The rapid flu test is a commonly obtained study. Your doctor inserts a brush-tipped device into your nasal passage to take a sample of the nasopharyngeal mucous secretions.  This is then analyzed for influenza viruses within minutes.  They may send it for confirmatory tests to the laboratory called influenza and B PCR. Depending on the lab, results return within hours to a couple of days.

If you have signs of dehydration or systemic toxicity, your doctor will likely order blood tests such as a comprehensive metabolic panel (CMP), complete blood cell count (CBC), urinalysis, and lactate level.  If you have respiratory symptoms suggestive of pneumonia, they will often obtain a chest x-ray.  Influenza and pneumonia frequently co-exist – influenza virus can produce viral pneumonia with bacterial co-infection.  Your doctor will evaluate for signs of pneumonia on the chest x-ray, such as areas of infiltrate or consolidation.

If pneumonia is diagnosed on chest x-ray, a repeat study should be performed in 4-6 weeks to ensure the consolidation has resolved.  If the consolidation has not improved, this could suggest an underlying mass such as lung cancer.

Influenza Prevention and Treatment

Influenza is prevented by avoiding exposure to individuals infected by the virus and through annual influenza vaccinations.  Recent studies have shown that the nasal influenza spray is not effective against the 2016-2017 influenza viruses.  The intramuscular influenza vaccine can prevent influenza infection and its complications. It is typically indicated in children, adults, and pregnant women.

The treatment of the flu depends on the clinical status of the patient.  Patients who are tolerating oral liquids and food without signs of significant toxicity can be managed as outpatients.  Your doctor will likely recommend rest and drinking plenty of fluids – this means at least 2L per day for most people.  Patients that present within 72 hours of flu symptoms may benefit from treatment with Tamiflu (oseltamivir).  This is an antiviral agent that may reduce the severity and duration of your symptoms.  Otherwise, the outpatient treatment of influenza virus infection typically includes:

Tylenol (acetaminophen) or NSAIDs (e.g., Advil, Naprosyn) are treatments for a sore throat, headache, body aches, and fever or chills.

Decongestant nasal sprays such as Afrin (phenylephrine) or Sinex (oxymetazoline) manages a runny nose or nasal congestions.  They may also recommend oral decongestants such as Sudafed (pseudoephedrine) – this should be avoided in patients with heart disease.

Severe cases of influenza may result in pneumonia, sepsis, respiratory failure, and shock.  Patients that are very ill typically require hospitalization with oxygen supplementation, intravenous fluids, and intravenous antibiotics.  Patients with severe breathing difficulty or respiratory failure may necessitate endotracheal intubation with respiratory support.


  1. Glezen WP. Clinical practice. Prevention and treatment of seasonal influenza. N Engl J Med 2008; 359:2579. -
  2. Havers F, Flannery B, Clippard JR, et al. Use of influenza antiviral medications among outpatients at high risk for influenza-associated complications during the 2013-2014 influenza season. Clin Infect Dis 2015; 60:1677. -
  3. (Accessed on January 03, 2015).


Influenza Medication List

The above information is an educational aid only. It is not intended as medical advice for individual conditions or treatments. Talk to your doctor, nurse or pharmacist before following any medical regimen to see if it is safe and effective for you.