Diphtheria cases have been reported in Australia for the first time this century. What is it and who is at risk?

Diphtheria spreads through close contact with infected individuals, but experts say the risk of an outbreak from the NSW cases is ‘almost zero’. Photograph: Hotli Simanjuntak/EPA

Diphtheria caused more deaths in Australia in the early 1900s than any other infectious disease. Thanks to diphtheria vaccines being rolled out nationally since 1932, it has almost disappeared, though it remains endemic in countries without organised vaccination programs.

It’s why two cases in young children being reported within two days, both in northern New South Wales, has caused alarm. An unvaccinated toddler is in intensive care with the disease. On Sunday the child’s six-year-old unvaccinated brother also tested positive, and is being treated in hospital as a precaution.