Influenza in humans

Flu viruses in the throat, artwork. © Science Photo Library

Influenza, also known as flu, is a respiratory illness caused by influenza viruses. It occurs globally and affects an estimated 5-10% of adults and 20-30% of children annually. Influenza viruses are categorised into types A, B, C, and D, with types A and B being the most significant in terms of human disease. Influenza C is less common and typically causes milder respiratory infections than influenza A and B, while influenza D is known to exclusively infect animals (mainly cattle) so far.

Seasonal influenza refers to the flu viruses that circulate annually, mainly during the winter months. There are two subtypes of influenza A viruses that are currently circulating among humans during the annual influenza epidemics (A(H3N2) and A(H1N1)pdm09) and two influenza B lineages (B/Victoria and B/Yamagata); B/Yamagata-lineage viruses however have not been detected since 2020. Seasonal influenza is a vaccine-preventable disease.

Zoonotic influenza viruses - such as avian (bird) flu and swine (pig) flu - are those that originate in animals and can, on occasion, cross over to infect humans. These instances are rare but can lead to severe disease and have the potential to cause a pandemic if the virus gains the ability to spread easily from person to person.

Transmission, symptoms and severity

Influenza viruses spread primarily through droplets when infected people cough, sneeze, or talk. These droplets can pass the mouths or noses of people who are nearby and possibly be inhaled into the airways. A person might also get the flu by touching a surface or object with the flu virus on it and then touching the mouth, the nose, or possibly the eyes.

Influenza symptoms can vary, ranging from none to mild, moderate, or severe. Common symptoms include a high temperature, cough, sore throat, stuffy or runny nose, muscle pain, headaches, and general fatigue. Severe outcomes of flu infection can result in hospitalisation or death, especially in high-risk groups like the elderly, very young children, pregnant women, and individuals with chronic health conditions.