Causes of respiratory tract infections
NHS Choices Medical Reference
Most upper respiratory tract infections are caused by viruses. There are more than two hundred different viruses that can cause a respiratory tract infection. However, the most common viruses belong to one of two groups:
The most common respiratory tract infection is a cold. The number of different viruses that can cause a cold is the reason why it is possible to have several colds, one after the other, with each one being caused by a different virus.
Most lower respiratory tract infections are also caused by viruses. The exception is pneumonia, which is usually caused by streptococcus pneumoniae bacteria.
How infections spread
Respiratory tract infections can be spread in several ways. If you have an infection such as a cold, tiny droplets of fluid containing the cold virus are launched into the air whenever you sneeze, cough or speak. If these are breathed in by someone else, they may also become infected.
Infections can also be spread through direct and indirect contact. If you have an infection such as a cold or the flu, and you touch your nose or eyes before touching someone else, you may pass the virus on to them. Alternatively, if you touch an object, such as a door handle, or telephone, the virus may be transferred to the object. If someone touches the object a short time later, and then touches their mouth, nose or eyes, they may become infected.
- Bacteria are tiny, single-celled organisms that live in the body. Some can cause illness and disease and some others are good for you.
- The immune system is the body's defence system, which helps protect it from disease, bacteria and viruses.