Infectious diseases are caused by infectious agents (bacteria, viruses, parasites and fungi and their toxic products). Many infectious diseases are also communicable diseases, meaning they can be passed from one person or animal to another. Transmission can occur directly (through contact with blood and bodily fluids) or indirectly (through contaminated food, water or surfaces) or by means of vectors (such as mosquitoes). Examples of these communicable diseases include measles (direct or indirect transmission), malaria (transmitted by mosquitoes), and chlamydia (direct transmission through sexual contact).

Throughout the 1900s, improved sanitation and new prevention and treatment options drastically reduced the burden of infectious diseases. Immunisation and vaccination is a key preventive measure against infectious and communicable diseases and has been highly successful at reducing infections from significant diseases. Australia’s high vaccination coverage, along with well-developed disease surveillance and response systems, led to Australia being declared polio-free in 2000, and achieving measles elimination in 2014.

Although the burden of infectious diseases in Australia is relatively small (1.7% of total burden in 2018) (AIHW 2021), most people will experience an infectious 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, as well as deaths. Some have developed resistance to antimicrobial agents, increasing the risk of more lengthy and complex treatment and poor outcomes (ACSQHC 2017). 

COVID-19 pandemic (novel coronavirus – SARS-CoV-2)

SARS-CoV-2 is a coronavirus which was first observed in late 2019 and causes the disease known as COVID-19. The World Health Organization (WHO) declared COVID-19 a pandemic (that is, the worldwide spread of a new infectious disease) on 11 March 2020. By 1 May 2022, there had been over 511 million confirmed cases worldwide and more than 6.2 million confirmed deaths (WHO 2022b), although the true numbers are likely to be considerably higher due to under-detection and gaps in vital registration coverage (Phipps et al. 2020; WHO 2022a).

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 COVID-19 in Australia (see Department of Health COVID–19 current situation). 

'Chapter 1 The impact of a new disease: COVID-19 from 2020, 2021 and into 2022’, 'Chapter 2 Changes in the health of Australians during the COVID-19 period’ and 'Chapter 3 Changes in Aboriginal and Torres Strait Islander people's use of health services in the early part of the COVID-19 pandemic’ in Australia’s health 2022: data insights look at how the pandemic has affected the health and health-related behaviour of Australians in general and Aboriginal and Torres Strait Islander people in particular.

  

In some cases, the illness caused by an infectious disease is mild and short-lived and medical care is not required or sought – for example, a cold. As a result, the prevalence of many infectious 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 Diseases Surveillance System (NNDSS).

Notifiable diseases

Notifiable diseases are a subset of infectious 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 (including COVID-19), and other bacterial diseases (see National notifiable disease list).

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

How common are infectious diseases?

Notifiable diseases

Almost 702,000 cases of notifiable diseases were reported to the NNDSS in 2021. Four diseases accounted for 89% of notifications to Australian health authorities in 2021:

  • COVID-19 – almost 471,000 notifications
  • chlamydia (a sexually transmissible infection) – almost 86,000 notifications
  • campylobacter (a gastrointestinal infection) – more than 37,000 notifications
  • varicella zoster (which causes chickenpox and shingles) – more than 33,000 notifications combined (Figure 1).

 

This figure presents a line graph showing the number of notifications for various notifiable diseases between 2009 and 2021. The reader can select all notifiable diseases or specific sub-types and individual diseases. The data show that the most commonly notified bloodborne disease was hepatitis C (unspecified), the most common gastrointestinal infection was campylobacteriosis, the most common STI was chlamydia, the most common vectorborne disease was Ross River virus, the most common vaccine-preventable disease was influenza, the most common zoonotic disease was Q fever and the most common other bacterial disease was tuberculosis.  The number of notifications for many diseases decreased during 2020 and 2021.

 

The impact of the pandemic on other infectious diseases

During the COVID-19 pandemic in 2020 and 2021, notifications of several other diseases were reduced (Bright et al. 2020).

  • Notifications of diseases usually acquired overseas or offshore, such as dengue, Chikungunya virus and malaria, fell considerably.

  • The number of cases of measles dropped from an average of around 155 cases per year between 2009 and 2019 to 25 in 2020 and 0 in 2021. Measles is highly infectious but is considered eliminated in Australia, meaning that local outbreaks can generally be linked to a case brought in from overseas.

  • Influenza, usually responsible for the most notifications in Australia each year, fell from a 5-year average of 163,000 notifications per year over 2015–2019 to 21,363 in 2020 and 731 in 2021. The majority of influenza notifications in 2020 occurred in the first 3 months of the year, before the first lockdowns began.

  • Notifications of chlamydia and gonorrhoea, which had been steadily increasing, were 16% and 22% lower, respectively, in 2021 compared with 2019.

  • Notifications of shigellosis (a bacterial gastrointestinal infection) were 85% lower in 2021 compared with 2019.

Public health measures put in place to control the pandemic, such as physical distancing, international and local travel restrictions, lockdowns, mask-wearing and handwashing, would have also affected the spread of other infectious diseases, particularly respiratory viruses (Sullivan et al. 2020). In addition, people may have been less likely than usual to seek medical care for relatively minor illnesses, leading to under-diagnosis and under-reporting for some diseases. It is difficult to determine the relative contribution of these various factors to the declines.  

Vaccine-preventable diseases

One key group among notifiable diseases is vaccine-preventable diseases. Many of these, including rubella (3 notifications in 2021), diphtheria (6 notifications in 2021) and tetanus (3 notifications in 2021) are rare in Australia, because 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.

Up until 2019, influenza, usually preventable by vaccination, accounted for the most notifications in Australia each year. Notifications had generally increased over time but annual totals fluctuated from year to year 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. As noted above, however, the number of influenza notifications decreased substantially during the COVID-19 pandemic.

Sexually transmissible infections

The number of notified sexually transmissible infections (STI) has generally increased over the last decade, though dropping during the COVID-19 pandemic, with chlamydia being the most commonly notified STI (almost 86,000 notifications in 2021). Varying prevention and control measures are used by public health authorities depending on the type of infection. Monitoring of the priority populations most commonly affected by STI allows targeted prevention programs to be designed.

An ongoing outbreak of infectious syphilis among young Indigenous Australian adults in Queensland, the Northern Territory, Western Australia and South Australia has contributed to increasing numbers of notifications over the last decade. Since January 2011 the outbreak has resulted in almost 4,500 notifications in these jurisdictions (Department of Health 2022). In general, new syphilis cases in Australia are diagnosed mainly in men who have sex with men in urban areas, or young Indigenous Australians in Remote or Very remote regions, although more recently increased numbers of cases have occurred among non-Indigenous women of reproductive age (15–44 years) in urban areas (Department of Health 2022).

STI notifications among Indigenous Australians are disproprotionately higher than in non-Indigenous Australians. Lack of access to culturally appropriate health services for testing and treatment, combined with the effect of social determinants, increase the risk of STI for Indigenous Australians (Kirby Institute 2018; Wand et al. 2016; Ward et al. 2020). Collection of information about testing for STI among Indigenous Australians aged 15–34 is being piloted in 2022 as part of the national Key Performance Indicators (nKPIs) for Indigenous-specific primary health care organisations (see AIHW 2022 for more information about the nKPI collection). 

The ongoing response to the rise in STI in Australia is being coordinated by the Department of Health through the National Blood Borne Viruses and Sexually Transmissible Infections Strategies. The next iteration of the Strategies for 2023–2030 is in development, with the overarching goal of eliminating blood borne viruses and STI as public health threats by 2030.

Non-notifiable diseases

Non-notifiable infectious 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. Information on hospitalisations and deaths from non-notifiable infectious diseases is presented in the following sections.

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

The impact of infectious diseases

Hospitalisations

In 2020–21, there were more than 324,000 hospitalisations for infectious diseases, of which 94% were for non-notifiable diseases. The hospitalisation rate for non-notifiable infectious diseases generally increased between 2006–07 and 2016–17, before stabilising at around 16 per 1,000 people (Figure 2). The rate dropped considerably during the following 2 years, falling to 14.2 per 1,000 in 2019–20 and 11.9 per 1,000 in 2020–21.
 

This figure presents a line graph with 2 lines for notifiable and non-notifiable diseases. The reader can select either hospitalisations or deaths, and numbers or crude rates. The data show that hospitalisations and deaths from non-notifiable diseases were considerably more common than from notifiable diseases, but that both hospitalisations and deaths decreased substantially in 2020 and 2021.

The most common causes of infectious disease hospitalisation across all years were lower respiratory tract infections (including pneumonia and bronchitis, but excluding laboratory-diagnosed influenza, which is notifiable). Lower respiratory infections had been generally increasing between 2000–01 (464 hospitalisations per 1,000 people) and 2018–19 (588 per 1,000), but this fell to 515 per 1,000 in 2019–20 and then to 390 per 1,000 in 2020–21 (Figure 3). If the trend observed between 2000–01 and 2018–19 had continued, rates of around 600 per 1,000 would have been expected in the following 2 years.

 

This figure presents a line graph with lines for each of the non-notifiable disease sub-types. The reader can select either hospitalisations or deaths, and numbers or crude rates. The data show that lower respiratory infections were the most common cause of hospitalisation and of deaths, but that numbers and rates decreased substantially in 2020 and 2021.

The hospitalisation rate for notifiable infectious diseases ranged between 0.9 and 2.1 per 1,000 people over the period 2000–01 to 2019–20 but dropped to 0.7 per 1,000 in 2020–21 (Figure 2). Influenza was the most common cause of notifiable disease hospitalisations in most years over the past decade, though highly variable from year to year, ranging from 2,185 to 30,808 hospitalisations (from 10 to 125 to hospitalisations per 1,000 people). In 2020–21, however, there were only 368 hospitalisations for influenza, a rate of 1.4 per 100,000 people. This is consistent with the fall in the number of notified cases described above.

In 2020–21 there were more than 4,700 hospitalisations where COVID-19 infection was recorded. As per Australian coding rules (IHPA 2021), in almost all cases COVID-19 was not specified as the principal diagnosis. Instead, the most common principal diagnoses recorded along with COVID-19 were viral pneumonia (24% of cases), ‘coronavirus infection’ (19%), and symptoms such as cough (9.9%), fever (6.6%) and breathing abnormalities (4.8%).

See Hospitals for more information on hospitalisation in Australia.

Deaths

In 2020, more than 5,800 deaths in Australia were attributed to infectious diseases, a rate of 23 per 100,000 people. Just over three-quarters (78%) of these deaths were attributed to non-notifiable diseases, the most common of which were lower respiratory infections (almost 2,500 deaths, 9.6 per 100,000) (Figure 2, Figure 3).

In 2020, there were 905 deaths with COVID-19 as the underlying cause (3.5 per 100,000 people). COVID-19 was the most common notifiable disease causing deaths in 2020. Influenza, which was the most common cause of notifiable disease deaths between 2014 and 2019, was recorded as the underlying cause of 54 deaths in 2020. A further 1,306 deaths with COVID-19 as the underlying cause were registered in 2021 (ABS 2022). More information on deaths from COVID-19 is provided in 'Chapter 1 The impact of a new disease: COVID-19 from 2020, 2021 and into 2022’ in Australia’s health 2022: data insights.  See Causes of death for more information on deaths in Australia.

Where do I go for more information?

For more information on infectious diseases, see:

References 

ABS (Australian Bureau of Statistics) (2022) COVID-19 Mortality in Australia: Deaths registered until 31 March 2022, ABS, Australian Government, accessed 19 May 2022.

ACSQHC (Australian Commission on Safety and Quality in Health Care) (2017) AURA 2017: Second Australian report on antimicrobial use and resistance in human health, ACSQHC, accessed 20 January 2022.

AIHW (Australian Institute of Health and Welfare) (2021) Australian Burden of Disease Study: impact and causes of illness and death in Australia 2018, AIHW, Australian Government, accessed 20 January 2022.

AIHW (2022) Aboriginal and Torres Strait Islander specific primary health care: results from the nKPI and OSR collections, AIHW, Australian Government, accessed 2 March 2022.

Bright A, Glynn-Robinson A-J, Kane S, Wright R and Saul N (2020) ‘The effect of COVID-19 public health measures on nationally notifiable diseases in Australia: preliminary analysis’, Communicable Diseases Intelligence, 44, accessed 30 March 2022.

Department of Health (2022) National syphilis surveillance quarterly report – 1 July to 30 September 2021, Department of Health website, accessed 30 March 2022.

IHPA (Independent Hospital Pricing Authority) (2021) Rules for coding and reporting COVID-19 episodes of care, IHPA, accessed 3 February 2022.

Kirby Institute (2018) HIV, viral hepatitis and sexually transmissible infections in Australia: annual surveillance report 2018, Kirby Institute, University of New South Wales, accessed 2 March 2022.

Phipps SJ, Grafton RQ and Kompas T (2020) ‘Robust estimates of the true (population) infection rate for COVID-19: a backcasting approach’, Royal Society Open Science, 7(11):1–12, doi:10.1098/rsos.200909.

Sullivan SG, Carlson S, Cheng AC, Chilver MBN, Dwyer DE, Irwin M, Kok J, Macartney K, MacLachlan J, Minney-Smith C, Smith D, Stocks N, Taylor J and Barr IG (2020) ‘Where has all the influenza gone? The impact of COVID-19 on the circulation of influenza and other respiratory viruses, Australia, March to September 2020’, Eurosurveillance, 25(47):1–6.

Wand H, Ward J, Bryant J, Delaney-Thiele D, Worth H, Pitts M and Kaldor JM (2016) ‘Individual and population level impacts of illicit drug use, sexual risk behaviours on sexually transmitted infections among young Aboriginal and Torres Strait Islander people: results from the GOANNA survey’, BMC Public Health, 16:1–9, doi:10.1186/s12889-016-3195-6.

Ward JS, Hengel B, Ah Chee D, Havnen O and Boffa JD (2020) ‘Setting the record straight: sexually transmissible infections and sexual abuse in Aboriginal and Torres Strait Islander communities’, Medical Journal of Australia, 212(5):205–207, doi:10.5694/mja2.50492.

WHO (World Health Organization) (2022a) The true death toll of COVID-19: estimating global excess mortality, WHO website, accessed 2 March 2022.

WHO (2022b) WHO Coronavirus (COVID-19) dashboard, WHO website, accessed 6 May 2022.