Seasonal influenza: epidemiology, etiology, and prevention

Seasonal influenza, also known as the flu, is an infectious disease caused by influenza viruses. It is characterized by fever, runny nose, sore throat, muscle pains, headache, coughing, and feeling tired. While usually mild to moderate, influenza can cause severe illness and life-threatening complications in many people. The disease is responsible for approximately 375,000 deaths annually, with 3-5 million cases reported each year.

Influenza viruses are classified into three types: Influenza A, B, and C. Influenza A is the most common and severe type, with influenza A subtype H1N1 being particularly noteworthy for causing the 1918 Spanish flu pandemic and the 2009 swine flu pandemic. Influenza B almost exclusively infects humans and is less common than influenza A. Influenza C causes mild illness in humans, dogs, and pigs.

The replication process of influenza viruses is a multi-step process, involving host cell invasion, genome delivery, viral protein and RNA production, assembly, and exit from the host cell. Influenza viruses bind through hemagglutinin onto sialic acid sugars on the surfaces of epithelial cells, typically in the nose, throat, and lungs of mammals, and intestines of birds. The virus then enters the cell, replicates, and releases new viral particles.

Influenza can be transmitted through respiratory droplets from coughing or sneezing and by touching surfaces contaminated with the virus. The risk of infection can be reduced by frequent hand washing, wearing surgical masks, and getting vaccinated. Annual vaccination is recommended for those at high risk, with the vaccine usually effective against three or four types of influenza.

Antiviral drugs such as oseltamivir can also be used to treat influenza, but their benefits in otherwise healthy individuals do not appear to be greater than their risks. No benefit has been found in those with other health problems. Furthermore, influenza can be a burden on healthcare systems and cause significant economic loss, with annual costs estimated at around $11.2 billion in the United States alone.

In conclusion, influenza is a contagious infectious disease caused by influenza viruses, with various types and subtypes. Seasonal influenza causes millions of infections and hundreds of thousands of deaths worldwide each year. Despite the availability of vaccination and antiviral drugs, influenza continues to be a major public health challenge. Prevention and control strategies, such as vaccination, handwashing, and mask-wearing, remain essential in reducing the spread of influenza and protecting vulnerable populations.