In general, most ARIs are caused by viruses. Common respiratory viruses include rhinovirus, respiratory syncytial virus (RSV), influenza virus, human coronavirus, SARS-CoV-2, and adenovirus, although others have been identified.[1] A viral ARI can predispose someone to a secondary bacterial respiratory infection, which is when a bacterial infection (e.g., bacterial pneumonia) occurs during or after an initial infection with a virus.[2] Other ARIs, such as pharyngitis (strep throat), sinusitis, otitis media, and pneumonia, can be caused by bacteria alone. Less often, an ARI may be caused by a fungus or parasite.[3]
Many ARIs are contagious and can be transmitted between people through respiratory droplets or direct physical contact. When the microorganism is inhaled or comes into contact with the mucosal lining of the nose or eyes, it infects the cells of the respiratory mucosa and begins to multiply and spread.[4] Both the infecting organism and the immune system’s response to the infection create the symptoms associated with an ARI.[5]
Many respiratory infections follow seasonal variations, meaning they occur more often during certain times of the year.[6]