What causes respiratory tract infections (RTIs)?

Respiratory tract infections (RTIs) can be caused by viruses, bacteria and (less often) fungi.

Most common respiratory tract infections such as colds, sore throats, sinusitis and laryngitis are caused by viruses.

One type of virus can cause more than one type of RTI. For example, the viruses that cause a cold can also cause:

Bacteria can also cause RTIs, including serious infections such as pneumonia and whooping cough (pertussis). Sometimes bacteria can cause a second infection after you have had an RTI caused by a virus.

Find out more about different respiratory tract infections.

Who is at risk of respiratory tract infections?

Anyone can catch a respiratory tract infection, but some people are at more risk either of catching an infection, or from the complications of infection, especially:

  • children younger than 5 years old, especially those born with heart or circulatory problems
  • people aged 65 years or older, especially those living in a nursing home
  • people with long-term health problems like type 1 or type 2 diabetes, heart or kidney disease
  • pregnant women
  • people who are very obese (Body Mass Index [BMI] of 35 or higher)
  • Aboriginal and Torres Strait Islander people
  • people with breathing problems due to asthma, cystic fibrosis, or chronic obstructive pulmonary disease (COPD, sometimes called emphysema)
  • people with conditions that affect breathing including multiple sclerosis, spinal cord injuries, and seizure disorders
  • people with weakened immune systems (e.g. due to HIV, cancer and some medicines)
  • homeless people
  • people who smoke.

Antibiotics only work against bacteria — so they won’t help if your infection is caused by a virus. However, people who are generally unwell, or who have an ongoing health condition like the ones listed above, are at greater risk of complications from an RTI. They are more likely to benefit from antibiotics than other people.

Read our leaflet about Antibiotics: Why some people need them and some don't.