What causes Respiratory Tract Infections (RTIs)?

Bacterial or Viral RTI?

Respiratory tract infections (RTIs) are mostly caused by either viruses or bacterium.

Most upper RTIs are caused by viruses, which cannot be treated with antibiotics (these only work on bacterial infections). Our immune system can fight off most viral infections by itself1.

Some lower RTIs like pneumonia are caused by bacteria. For the RTIs caused by bacteria, if your immune system is having difficulty fighting off the infection your doctor may prescribe a course of antibiotics1.

Virus: A virus is a microscopic organism that invades living cells in order to reproduce, and that causes disease in people2.

Bacteria: Bacteria are tiny, single-celled organisms that live in the body. Some can cause illness and disease3.

Immune system: The immune system is the body’s defense system, which helps protect it from disease, bacteria and viruses3.

How do respiratory tract infections spread?

The most common RTIs are spread through the coughs and sneezes of someone with an infection4. This is because when we sneeze or cough, tiny droplets of fluid are sprayed into the air. If these are breathed in by someone else, they may catch the infection3.

Infections can also spread through direct or 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 to them. Touching a door handle that’s been touched by someone with an infection also means you might catch it3

This means a very efficient way to prevent spreading infections is to practice good hygiene.

Do not hesitate to consult your doctor if you want to learn more about the causes of the RTI or for practical advice on how to limit risks of getting one.    


  1. Health Navigator New Zealand. Respiratory tract infections. Available at: https://www.healthnavigator.org.nz/health-a-z/r/respiratory-tract-infections/. Accessed October 14, 2020.
  2. Centers for Disease Control and Prevention. Vaccines & Immunizations, Glossary. Available at: https://www.cdc.gov/vaccines/terms/glossary.html. Accessed October 14, 2020.
  3. Health Service Executive. Respiratory tract infection. Available at: https://www.hse.ie/eng/health/az/r/respiratory-tract-infection/causes-of-respiratory-tract-infections.html. Accessed October 14, 2020.
  4. National Health Service. Respiratory tract infections (RTIs). Available at: https://www.nhs.uk/conditions/respiratory-tract-infection/. Accessed October 14, 2020.