Influenza is an acute viral respiratory infection caused by influenza viruses, primarily types A and B. Influenza A is known for causing pandemics due to its high susceptibility to antigenic variation.

Symptoms: Influenza typically presents with the abrupt onset of fever, cough, chills or sweats, myalgias, malaise, headache, and sore throat. Symptoms can range from mild to severe, with some cases leading to complications such as pneumonia, acute respiratory distress syndrome, and multiorgan failure, particularly in older adults and those with chronic health conditions.