Stochastic models indicate rapid smallpox spread and mass mortality of Indigenous Australians after colonial exposure

The impact of smallpox (variola) on Aboriginal communities in Australia beginning in 1789 was catastrophic and continues to cause intergenerational trauma. Historically biased perspectives and contemporary misinformation of the disease’s introduction and spread impede modern-day truth-telling and subsequent reconciliation and national healing. Understanding whether the disease entered and spread from pre-colonial Makassan (Indonesian) trade along the northern coast, or from the First Fleet’s arrival in south-eastern Australia in 1788, is necessary to estimate demographic impact. We developed stochastic, multi-patch epidemiological models supported by systematic evaluation of historical observations to test hypotheses regarding possible disease entry points, spread rate, and demographic impacts. Our models suggest that entry of the disease was in south-eastern Australia. No successful simulations reached Sydney from a northern entry, even under ideal conditions and with higher-than-probable infection rates. Our model predicts time from initial entry of the disease to its extinction took between 1,200 and 1,400 days (< 4 years). Due to a high mortality rate, it appears to have been limited to the south-eastern coastal margins of Australia and along major intersecting river systems such as the Murray and Lachlan Rivers. There is no evidence that the 1789 epidemic was Australia-wide. Assuming a 60% lethality based on global data, the loss of 40,000–240,000 people would have occurred in these regions in < 4 years. While catastrophic to traditional Indigenous lifeways in the southeast, the disease also provided the catalyst for population decline and marginalisation of Indigenous people in the face of expanding European populations. It seems unlikely that other parts of Australia were affected by the initial epidemic, and we recommend revisiting previous assumptions of the subsequent impacts to Indigenous societies by other diseases and frontier violence. While there is evidence for the presence of variola matter in early colonial Sydney, we cannot yet confirm that the disease was spread deliberately. We warn readers that the content of this study is confronting and possibly distressing.