One of the mysteries of the Covid-19 crisis is how the disease seemed to bubble up out of nowhere in Italy at the end of February – at a time when it seemed to be under control in China. In spite of local quarantines and the isolation of individual patients, the epidemic quickly took hold. We have subsequently had reports of patients infected all over Europe who were possibly infected in mid-January.

But now a study has emerged that suggests the disease was already circulating in Northern Italy before Christmas. A team from the Institute for Public Health in Rome studied 40 wastewater samples collected from sewage works near Milan, Turin and Bologna between October last year and February this year. In 15 of them, they detected the presence of Sars-CoV-2, the virus that causes Covid-19. Eight of these positive samples came from before Italy’s first native case of Covid-19 was diagnosed on 21 February. The earliest of the positive samples were collected in Milan and Turin on 18 December 2019. The team also checked samples collected between September 2018 and June 2019, which proved negative.

The earliest sample appears a fortnight before the disease was even identified in China. It was, however, after the opening of the Austrian ski resort Ischgl, which began its season with an open-air concert on 30 November and which has been identified as the possible seat of the epidemic in Europe. Chinese media reports have traced the earliest case in China to 17 November, although a University of Cambridge team mapping the early spread of the virus have claimed that it could have infected its first human as early as 13 September.

If the virus was present in Italy before Christmas, it shows why it has proved so difficult to contain it. Italy, as well as other European countries, has been criticised for their failure to isolate the first cases. But if the disease was in general circulation several weeks earlier than the first cases were identified then the authorities really had no chance of shutting down the epidemic. It might also explain why some countries have had such a different experience with Covid-19 compared with others. Some are praised for their low numbers of cases and deaths, but the real determinant of how different countries coped is less public policy and the effectiveness of healthcare systems – it is the extent to which the virus was able to become established prior to anyone knowing what it was.

What it shows is that the viruses that damage us most are not those that are the most deadly – they can quickly be identified and brought under control by isolating patients. The greater danger comes from a virus, like Sars-CoV-19, which is capable of killing a small proportion but causes no symptoms whatsoever in four out of five people it infects. It is like an army, 80 per cent of whose soldiers are invisible. How easy it would be to establish a beachhead if it took opposing gunners a month to realise that your troops had waded onto their territory. <//>