Legionnaires' disease - what is it and why have case numbers surged?

SBS News In Depth - A podcast by SBS

Victorian health authorities say they've identified the source of the state's deadly legionnaires' outbreak.So far, there are 77 confirmed patients, and an additional nine suspected cases. Two people have died as a result of the outbreak - Victoria's worst in two decades. So what exactly is legionnaires disease and what caused the surge in cases?