Coronavirus is evolving. Whether it gets deadlier or not may depend on us | Laura Spinney

There's now evidence that ignoring social distancing rules could help more lethal strains of Covid-19 to win out

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Letting the virus that causes Covid-19 circulate more-or-less freely is dangerous not only because it risks overwhelming hospitals and so endangering lives unnecessarily, but also because it could delay the evolution of the virus to a more benign form and potentially even make it more lethal.

Though the data is still sketchy and the measures crude, this effect may already be influencing the difference in death rates between Sweden - which took a relaxed approach to containment until recently - and Norway, whose measures have been much stricter. Sweden has more than three times as many deaths per 100 cases as its neighbour.