Australia is experiencing its worst diphtheria outbreak in decades, with more than 220 confirmed cases reported since January, according to reporting that draws on public health data and expert commentary. Multiple sources link the outbreak to declining vaccination coverage over time, including people who are under-vaccinated or not fully protected due to waning immunity and gaps in routine immunisation. Public health experts say the resurgence of diphtheria is driven by insufficient population protection, allowing the infection to spread more easily when outbreaks occur. The reports note that diphtheria is a preventable bacterial disease, and that targeted vaccination and catch-up immunisation are central to controlling transmission and reducing future case numbers. While the outbreak is described as historic in scale, sources emphasise that it can be mitigated through improved uptake of diphtheria vaccines in affected communities and among those at higher risk. Overall, the accounts converge on the same core explanation: lowered vaccine protection across parts of the population is contributing to the current increase in cases and the severity of the outbreak.