The announcement that Donald Trump has tested positive for the Sars-Cov-2 virus has rocked the US barely a month out from the presidential election. Much will now depend on how his condition evolves.

A striking feature of Covid-19 has been its unpredictability. Infection can cause symptoms ranging from the pneumonia that characterised the disease when it emerged in China, to problems with the heart and circulation, or the brain and nervous system. In other cases, people infected with the virus — including those in their seventies such as Mr Trump — can recover without suffering any symptoms, or nothing worse than a temporary loss of smell and taste.

Mr Trump is 74. On average, someone in their mid-seventies who tests positive for coronavirus has about a 20 per cent chance of requiring hospitalisation and a 5 per cent chance of dying from Covid-19 in countries with good medical systems.