Infectious diseases (also known as communicable diseases) are caused by infectious agents and can be passed from one person or animal to another. Transmission can occur directly (through contact with bodily discharge), indirectly (for example, by sharing a drinking glass) or by means of vectors (such as mosquitoes). They are caused by bacteria, viruses, parasites or fungi or their toxic products. Examples of these communicable diseases include coronavirus, malaria, influenza and chickenpox.

Throughout the 1900s, improved sanitation and new prevention and treatment options drastically reduced the burden of communicable diseases. Immunisation and vaccination is a key preventive measure against communicable diseases and has been highly successful at reducing infections from significant diseases such as polio and tetanus.

Although the burden of infectious diseases in Australia is relatively small (2.0% of total burden) (AIHW 2019), most people will experience an infection from a communicable disease during their lifetime—for example, a common cold or a stomach bug. Many infectious diseases have the potential to cause significant illness and outbreaks. Some have developed resistance to antimicrobial agents, increasing the risk of more lengthy and complex treatment and poor outcomes (ACSQHC 2017). 

Outbreak: COVID-19 pandemic (novel coronavirus—SARS-CoV-2)

SARS-CoV-2 is a coronavirus which was first observed in Wuhan City, China, in December 2019 and causes the disease known as COVID-19. It has animal origins (Huang et al. 2020) and infections in humans have not been observed previousnsly. The World Health Organization declared COVID-19 a pandemic (that is, the worldwide spread of a new infectious disease) on 11 March 2020.

Early symptoms of COVID-19 include fever, fatigue and respiratory symptoms including coughing, sore throat and shortness of breath. The disease ranges from mild illness to serious illness possibly causing death. SARS-CoV-2 is related to other coronaviruses, such as those causing severe acute respiratory syndrome (SARS) and middle east respiratory syndrome (MERS), which have previously caused serious outbreaks.

The Commonwealth Department of Health’s website provides updated information about the evolving situation in Australia (see Department of Health COVID–19 current situation). 

While the full impact of COVID-19 on the health of Australians will not be known for some time, the article ‘Four months in: what we know about the new coronavirus disease in Australia’ in Australia’s health 2020: data insights takes a look at the early days of the disease in Australia. It compares Australia with other countries and compares COVID-19 with other diseases in Australia.     

Often, the illness caused by an infectious disease is mild and short-lived and medical care is not required or sought. As a result, the prevalence of many communicable diseases is difficult to measure. To assist in understanding their impact, certain infectious diseases are notifiable conditions. When a diagnosis is made of one of these diseases, a report is made to health authorities. Notification means that trends in the number and characteristics of cases can be monitored over time from a consistent and comparable data set. Outbreaks can be detected in a timely way so that interventions can be implemented to prevent or reduce transmission. Monitoring, analysis and reporting on notifiable diseases occurs nationally via the National Notifiable Disease Surveillance System (NNDSS).

Notifiable diseases

Notifiable diseases are a subset of communicable diseases. Legislation requires that each detected case is reported to state and territory health departments. Notifiable diseases include bloodborne diseases, gastrointestinal diseases, sexually transmissible infections, vaccine-preventable diseases, vectorborne diseases, zoonoses, listed human diseases, and other bacterial diseases (see Department of Health list of Australian notifiable diseases).

This page highlights the impact of infectious diseases in Australia, both notifiable and non-notifiable. 

How common are notifiable infectious diseases?

More than 593,000 cases of notifiable diseases were reported to the NNDSS in 2019. Four infectious diseases accounted for 82% of these notifications to Australian health authorities in 2019:

  • influenza—more than 313,000 notifications
  • chlamydia—almost 103,000 notifications
  • campylobacter (a gastrointestinal infection)—almost 36,000 notifications
  • gonorrhoea—more than 34,000 notifications.
     

This line graph shows that the most commonly notified communicable disease between 2009 and 2017 for bloodborne diseases was hepatitis C (unspecified), for gastro related diseases it was campylobacteriosis, for bacterial diseases it was tuberculosis, for STIs it was chlamydial infections, for vectorborne diseases it was ross river virus, for vaccine preventable diseases it was influenza and for zoonoses it was Q fever. 

Vaccine-preventable diseases

One key group among notifiable diseases is vaccine-preventable diseases. Many of these, including rubella (22 notifications in 2019), diphtheria (7 notifications in 2019) and tetanus (3 notifications in 2019) are rare in Australia, as a result of Australia’s high immunisation rates (see Immunisation and vaccination). For some diseases such as pertussis (whooping cough) and measles, the number of notifications can increase during outbreak periods because people with low or no immunity can be infected.

Influenza, usually preventable by vaccination, accounts for the most notifications in Australia. Overall influenza notifications have generally increased over time but annual totals have fluctuated from year to year. There were more than 313,000 notifications in 2019, a substantial increase from 2018 (nearly 59,000 notifications). The number of influenza notifications changes depending on the particular type of influenza circulating in the population, and on factors such as the amount of laboratory testing of unwell people, or the types of tests used. The extent of under-notification and trends in notifications of influenza can change from year to year. Other surveillance systems are also used to determine trends in influenza to help understand the relative impact of the illness on society in Australia (Department of Health, 2019a).

The number of notifications of rotavirus, shingles and chickenpox have also risen recently. It is difficult to determine how much of the increase is due to improved diagnosis and notification and how much reflects a real increase in the number of infected people.

Sexually transmitted infections

Varying prevention and control measures are used by public health authorities depending on the type of infection or disease. Monitoring of the population groups affected by sexually transmitted infections (STIs) allows targeted prevention programs to be designed. The number of notified STIs has increased over the last decade, with chlamydia being the most commonly notified (more than 102,000 notifications in 2019). Gonorrhoea notifications have increased, but infections continue to affect the same groups: Aboriginal and Torres Strait Islander males and females in Remote or Very Remote areas, and non-Indigenous males in metropolitan settings.

Similarly, the number of notifications of infectious syphilis have increased in the last decade. New syphilis cases are diagnosed mainly in men who have sex with men in urban areas, or young Indigenous Australians in Remote or Very Remote regions. Part of the increase in numbers can be attributed to an ongoing outbreak among Indigenous Australians in Northern Australia, accounting for just over 3,000 notifications since January 2011 (Department of Health 2019b; Kirby Institute 2017).

How common are non-notifiable infectious diseases?

Non-notifiable communicable diseases are not routinely monitored, though their impact can be tracked through assessing presentations to hospital, or through mortality data. These data sources capture the small proportion of people who have severe illness, causing hospitalisation or resulting in death.

Classifying non-notifiable infectious diseases

The non-notifiable infectious diseases are broadly categorised based on the International Statistical Classification of Diseases and Related Health Problems, 10th Revision, Australian Modification (ICD-10-AM) codes for hospitalisations and International Statistical Classification of Diseases and Related Health Problems, 10th Revision (ICD-10) for deaths. The categories include other gastrointestinal infections, upper respiratory tract infections, lower respiratory tract infections, other meningitis and encephalitis, trachoma, abscess causing pneumonia*, otitis media, unspecified viral hepatitis, and other infections. 

*not listed as a separate category in mortality coding

Treatment and management

Hospitalisations

In 2017–18, 89% of the nearly 447,000 hospitalisations for communicable diseases were for non-notifiable diseases. From 2000–01 to 2017–18, the hospitalisation rate for non-notifiable communicable diseases peaked at 15.3 per 1,000 in 2016–17 (nearly 400,000 hospital separations). In 2017–18, the most commonly diagnosed infectious cause of hospitalisation was lower respiratory tract infections (such as pneumonia and bronchitis), accounting for almost 145,000 hospitalisations.

Influenza was the most common cause of hospitalisation for all notifiable diseases in 2017–18, with 62% of nearly 50,000 hospitalisations attributed to it. The hospitalisation rate per 1,000 population for notifiable communicable diseases has ranged between 0.8 in 2006–07 and 1.8 in 2017–18 (Figure 2).

See Hospital care.
 

This line graphs shows that in 2018, the highest number of hospitalisations for non-notifiable infectious communicable diseases was due to lower respiratory infections with 144,736 hospitalisations, while unspecified viral hepatitis had the lowest number with 98 hospitalisations. In 2018, the highest number of deaths for non-notifiable communicable diseases was due to lower respiratory infections with 3,403 deaths, while 88 deaths were due to upper respiratory tract infections.

Impact

Deaths

In 2018, almost 6,000 deaths in Australia were attributed to communicable diseases. Between the years 2000 and 2018 around 99,000 deaths were attributed to communicable diseases. The vast majority (89%) of the deaths caused by infections were attributed to non-notifiable diseases, the most common of which were lower respiratory infections caused by various bacteria and viruses (Figure 3). The death rate per 100,000 population due to non-notifiable communicable diseases ranged from 13 in 2009 to 24 in 2003 (Figure 4).

This line graph shows that the hospitalisations rates for non-notifiable communicable diseases have increased from 11.8 per 1,000 population in 2000–01 to 15.1 per 1,000 population in 2017–18. The rate for notifiable communicable diseases increased from 1 per 1,000 population in 2000–01 to 2 per 1,000 in 2017–18. 

Of the 10,700 deaths attributed to notifiable diseases between 2000 and 2018, the most common causes of death were influenza (more than 3,300) and chronic hepatitis C infection (almost 1,500). It is likely that the true impact of influenza is underestimated in mortality data as many people who died would not have been tested for influenza.

See Causes of death.
 

This line graph shows a steady decline in mortality rates for non-notifiable communicable diseases, from 22.4 per 100,000 population to 17.9 per 100,000 population. The lowest mortality rate was seen in 2009 with a rate of 13.1 per 100,000. There was an increase in the mortality rate for notifiable communicable diseases, from 2.3 per 100,000 population in 2000 to 5.7 per 100,000 population in 2017. 

Where do I go for more information?

For more information on infectious and communicable diseases, see:

References

ACSQHC (Australian Commission on Safety and Quality in Health Care) 2017. AURA 2017: Second Australian report on antimicrobial use and resistance in human health. Sydney: ACSQHC.

AIHW (Australian Institute of Health and Welfare) 2019. Australian Burden of Disease Study: impact and causes of illness and death in Australia 2015. Australian Burden of Disease series no. 19. Cat. no. BOD 22. Canberra: AIHW.

Department of Health 2017 2017 Influenza season in Australia: a summary from the National Influenza Surveillance Committee. Canberra: Department of Health. Viewed December 2017.

Department of Health 2018. National notifiable diseases surveillance system, 2018. Canberra: Department of Health. Viewed January 2018.

Department of Health 2019a. Australian influenza surveillance report, No. 12, 2019, 23 Sept-6 October. Canberra: Department of Health. Viewed December 2019. 

Department of Health 2019b. Infectious syphilis outbreak. 20 Nov 2019. Viewed December 2019.

Huang C, Wang Y, Li X, Ren L, Zhao J, Hu Y, et al. 2020. Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China. The Lancet 2020 Feb 15; 395 (10223):497–506.

Kirby Institute 2017. National blood-borne viruses and sexually transmissible infections surveillance and monitoring report, 2017. Sydney: The Kirby Institute, UNSW Sydney.