A respiratory infection is an inflammation of the mucous membranes in your respiratory system (nose, mouth and airways) caused by a virus or bacteria. The air you breathe can contain all sorts of pathogens, including viruses and bacteria – such as Legionella or pneumococci bacteria. Common viruses that cause respiratory infections include the influenza virus (flu), the coronavirus SARS-CoV-2 (COVID-19) and the respiratory syncytial virus (RSV infections).