What is Influenza?

Influenza is a contagious respiratory illness. It is spread via respiratory droplets, meaning that a person who is ill with the flu can pass it on to others by coughing or sneezing. It usually takes between one and four days for a person who has been exposed to influenza to develop an infection, and symptoms can last for one to two weeks.

Influenza’s presence within the population fluctuates over the course of the year, with “flu season” typically beginning near the end of the year, in October, peaking in the mid-winter (usually in February), and continuing into the springtime, sometimes lasting as late as May. However, despite its seasonality, it is possible to contract influenza at any time of the year.

What Causes Influenza?

Influenza is caused by several different viruses that are in the influenza family. Each year, flu season is defined by a few predominant strains of the influenza virus. These viral strains vary from year to year and change over the course of time.

What Are the Symptoms of Influenza?

The experience of influenza can vary from person to person, however, the most commonly experienced symptoms include the following:

  • Fever and/or chills
  • Body aches and muscle aches
  • Cough
  • Sore throat
  • Runny or stuffy nose
  • Headache
  • Fatigue
  • Appetite loss

Some versions of influenza can also cause digestive symptoms such as nausea, vomiting, abdominal pain, or diarrhea; however, these symptoms are less common.

Who Gets Influenza?

People of all ages and backgrounds are at risk of getting influenza; however, the following factors make you more vulnerable to contracting an influenza infection:

  • Being of a very young or very old age
  • Being immunosuppressed, whether from illness, certain medications, or pregnancy
  • Smoking
  • Having lung disease, such as asthma or COPD, or other types of chronic disease
  • Not getting vaccinated against the flu
  • Not practicing respiratory hygiene measures, such as frequently washing your hands, not touching your face, and keeping your distance from those with respiratory symptoms
  • Being in a high-risk environment where viral outbreaks are likely, such as a nursing home, hospital setting, or incarcerated setting

It is also important to note that having influenza during one flu season does not protect you from having it again the next year, or even from getting sick again within the same year from a different strain of the virus.

How is Influenza Diagnosed?

Influenza can be diagnosed in a number of different ways. Your healthcare provider may be able to diagnose you clinically, based on your history and physical exam, particularly if you have been around others who are known to have had influenza.

Your healthcare provider may conduct a rapid influenza test, which is a nasal swab that can check for the presence of viral particles in your nasal passageway. Influenza tests cannot tell with 100 percent certainty whether or not you have influenza, so results are always paired with the broader clinical picture. Viral cultures and other types of diagnostic tests may also be used to diagnose influenza, particularly if you or a loved one require hospitalization.

Your healthcare provider may also use the following evaluations to determine the extent of your influenza illness: 

  • Vital signs (oxygen level, blood pressure, respiratory rate, heart rate, and temperature)
  • X-ray or CT of the chest
  • EKG
  • Blood work
  • Respiratory Function Testing

What Are the Complications of Influenza?

When your body is fighting an influenza infection, your immune system is vulnerable to other insults as well. Complications of influenza include the following:

  • Ear infections (especially in children)
  • Sinus infections
  • Bronchitis
  • Bacterial pneumonia
  • Sepsis
  • Encephalitis or meningitis
  • Heart inflammation (myocarditis)
  • Muscle inflammation (myositis or rhabdomyolysis)
  • Worsening of underlying chronic medical conditions