A cold is a type of upper respiratory tract infection (URTI) , which is the medical term used in healthcare. The upper airways include the nose, pharynx and larynx, while the lungs belong to the lower airways. If an infection spreads there, it can cause bronchitis or pneumonia. At that point it is referred to as a lower respiratory tract infection.
Many viruses can cause a cold
Rhinoviruses are the most common cause and is responsible for more than half of all upper respiratory tract infections. Rhinoviruses are a large family of viruses, divided into three subgroups, with up to 150–160 different types in total. Other viruses that can cause a cold include coronavirus, adenovirus, parainfluenza virus and RSV (respiratory syncytial virus), all of which can produce the symptoms we call a cold. The coronavirus referred to here is not the same as SARS-CoV-2, which caused COVID-19, but rather a family of different coronavirus types that includes the pandemic virus.
Even viruses not normally classified as respiratory viruses, such as measles virus or enterovirus, can cause cold-like symptoms when they infect the airways.
Newly discovered cold viruses
In recent decades, research has also identified new viruses. Human metapneumovirus was discovered in 2001 by researchers in the Netherlands. It later emerged that the virus had been circulating for at least 65 years, and that almost all children have been infected by the age of five. In 2005, human bocavirus was identified, another virus shown to cause cold symptoms in children. All indications suggest that researchers will continue to identify further viruses in the future.
Why you never become immune to colds
Because so many different viruses can cause a cold, recovering from one does not make you immune to all colds. Overall, a cold represents an entire spectrum of infections, with different origins but similar symptoms. This is one of the key reasons colds are so common and so difficult to avoid.
