Influenza (flu) is a universally common epidemic illness caused by several subtypes of type A or by type B influenza virus. (Type C influenza virus exists and produces a mild respiratory illness but is not believed to cause epidemics.) The most common subtypes of influenza A are H1N1 and H3N2. A subtype of avian influenza A that emerged in Asia (H5N1) produces infrequent but often fatal human illness. A swine-origin influenza A (H1N1) virus outbreak, beginning in March and April of 2009, reinforces that interspecies transmission of human and animal influenza A viruses and subsequent reassortment are possible and occur, resulting in new viruses that may spread to humans and even easily between humans.
Current flu season: Per the Centers for Disease Control and Prevention (CDC), in the week ending January 14, 2023, seasonal influenza activity continues to decline across the United States. Hospitalization rates are highest in adults 65 years and older and children 0-4 years. Influenza A strongly predominates over influenza B, with most cases attributed to influenza A(H3N2) but some cases attributed to influenza A(H1N1). Coinfection with influenza A or B viruses and SARS-CoV-2 can occur.
Influenza is highly contagious and is spread by aerosol droplets. The incubation period is 1-4 days, but it becomes contagious 1 day prior to the onset of symptoms. The mortality rate of influenza is low but tends to be higher in the elderly and the immunocompromised. Flu activity usually peaks in February.
Influenza presents with classic flu-like illness consisting of the sudden onset of fever, malaise, sore throat, nonproductive cough, myalgias, headache, and nasal congestion. Chills are common, as are nausea and vomiting in children. After 48 hours, cough may increase and produce sputum. There may be associated dyspnea and/or mild-to-moderate pleuritic chest pain. Upon physical examination, unilateral or bilateral inspiratory rales may be appreciated or diminished breath sounds. Pregnant individuals are at increased risk for severe illness from influenza.
Most viruses that affect the respiratory tract can cause a rash, flu included (see viral exanthem).
The most common pulmonary complication of influenza is secondary bacterial pneumonia. This diagnosis can be determined by patient history. This can occur up to 2 weeks after the initial symptoms and includes recurrence of fever, chills, pleuritic chest pain, and productive cough. Many bacteria may be the culprit, but the most common are pneumococci. Staphylococcus aureus has also been implicated in children; there has been an increase in the number of deaths in which both influenza and pneumonia or bacteremia due to S aureus were identified.
Primary viral pneumonia is the complication responsible for the most influenza-related deaths. Those with pre-existing cardiopulmonary disease or who are pregnant are at the greatest risk. The initial clinical presentation is the same, but dyspnea increases in severity. Productive cough may be blood tinged. Massive hemoptysis has been reported. When severe, there may be profound respiratory distress with tachypnea, tachycardia, and cyanosis. Rales and wheezes will spread throughout the chest from the lower lung.
The Spanish flu pandemic of 1918 was particularly virulent, killing over 20 million people worldwide. With present day biotechnology, it would be possible to produce an influenza virus weapon with traits of both the H5N1 and the 1918 influenza viruses. As a weapon, influenza could be released as an aerosol. The mortality of a designer influenza containing genes from H5N1 and/or the 1918 Spanish flu would presumably be much higher than for naturally occurring influenza. The initial symptoms produced by many of the biological weapons are nearly identical to those produced by influenza ("flu-like illness").
ICD10CM: J10.1 – Influenza due to other identified influenza virus with other respiratory manifestations
SNOMEDCT: 6142004 – Influenza
Differential Diagnosis & Pitfalls
Many conditions that may be caused by an act of biological terrorism have an initial presentation very similar to influenza. These include: