evidence: Nearly 100 years ago, in 1918, the world experienced the greatest tidal wave of death since the Black Death, possibly in the whole of human history. We call that tidal wave the Spanish flu, and many things changed in the wake of it. One of the most profound revolutions took place in the domain of public health. [...] Flu is caused by a virus, but virus was a novel concept in 1918, and most of the world’s doctors assumed they were dealing with a bacterial disease. This meant that they were almost completely helpless against the Spanish flu. They had no flu vaccine, no antiviral drugs, not even any antibiotics, which might have been effective against the secondary bacterial infections that killed most of its victims (in the form of pneumonia). Public health measures such as quarantine or the closing of public meeting places could be effective, but even when they were imposed this often happened too late, because influenza was not a reportable disease in 1918. This meant that doctors weren’t obliged to report cases to the authorities, which in turn meant that those authorities failed to see the pandemic coming. [...] Laura Spinney is a science journalist and novelist based in Paris.