Symptoms of Influenza

Influenza, commonly known as the "flu," is an acute respiratory illness caused by the influenza virus. Based on the core composition of the influenza virus, it is classified into types A, B, and C. Type A and type B mainly cause influenza in humans. Type C can also rarely infect humans, but its symptoms are usually mild.

Influenza occurs every winter in 10–20% of the population due to antigenic drift in the influenza virus. The global pandemics of type A influenza that occur every 10–40 years are caused by antigenic shift and pose a major threat to humanity. In fact, the novel influenza that began in April 2009 in the United States and Mexico was the first pandemic of the 21st century caused by a shift in the H1N1 virus and caused major damage worldwide.

Symptoms

1. Clinical Findings Influenza is an acute febrile respiratory illness characterized by the sudden onset of respiratory symptoms such as cough and sore throat, along with systemic symptoms such as high fever, chills, headache, muscle aches, or fatigue. Because its symptoms are very similar to those of the common cold caused by various respiratory viruses that are common in winter, differential diagnosis is difficult. However, influenza and the common cold are different diseases. Unlike the common cold, influenza can cause fatal complications, and because antiviral treatments and effective vaccines are available, differential diagnosis is necessary.

The clinically important point in distinguishing it from the common cold is that, unlike the common cold, influenza begins suddenly, so the exact time when the high fever started can be remembered. In addition, respiratory symptoms such as cough and sore throat appear as systemic symptoms improve, and in some patients the cough lasts more than one week and may be accompanied by chest pain. If high fever persists and symptoms such as cough, yellow sputum, and shortness of breath appear, the development of lower respiratory complications such as pneumonia should be suspected. In uncomplicated influenza, acute signs and symptoms usually improve over 2–5 days, and most people recover almost completely within one week, but the cough can last for several months. In a very small number of people, such as older adults, weakness or lethargy may persist for several weeks.

2. Complications The main causes of death associated with influenza are respiratory complications and worsening of underlying cardiopulmonary disease. Common upper respiratory complications in children include croup, acute sinusitis, and acute otitis media. Lower respiratory complications such as influenza viral pneumonia, exacerbation of chronic obstructive pulmonary disease, and bacterial pneumonia due to secondary infection may require hospitalization and can even lead to death.

Patients with chronic illnesses are at increased risk not only of influenza-related complications but also of death due to worsening of their existing conditions. In the elderly, influenza can cause dehydration and worsen underlying medical conditions such as heart failure, asthma, and diabetes, leading to progressive deterioration of cardiovascular, pulmonary, or renal function. For example, a patient with well-controlled angina may progress to myocardial infarction, a patient with asthma may develop acute attacks and become short of breath, or a patient with diabetes may suddenly have rising blood sugar levels. Such rapid worsening of an existing chronic disease can lead to hospitalization and even unexpected death.

So far, we have explained the symptoms of influenza. In the next part, we will look at the diagnosis and treatment of influenza.

Source: Korea Disease Control and Prevention Agency, National Health Information Portal