Stock markets across the world have SLOTXO dropped on fears that increasing numbers of Covid cases will hamper economic recovery.
In Asia, Japan's benchmark Nikkei 225 index opened down by more than 1% to hit its lowest level in six months.
In New York, the Dow Jones Industrial Average tumbled by more than 2%, while major indexes in the UK and mainland Europe also fell.
New cases are rising in some countries, fuelled by the Delta variant.
Japanese shares are now down for a fifth session in a row over concerns about the spread of the highly contagious Delta variant and that the Tokyo Olympics could worsen the health crisis.
Meanwhile, businesses in the UK have described how they are cutting opening hours, closing shops or may have to reduce production as employees are forced to self-isolate after being alerted by the NHS Covid-19 app.
"The big concern for the market is whether we are going to see a slowdown in the global economic recovery," said Russ Mould, investment director at AJ Bell.
"Optimism which had been on the horizon just a few weeks ago has again been obscured by dark clouds, especially with the snap decision to enforce a 10-day quarantine for arrivals from France," said Susannah Streeter, senior investment and markets analyst at Hargreaves Lansdown.
"With some scientists warning that infections could reach 200,000 a day by September, there is now a feeling that the UK could be staring at a fresh autumn lockdown," she added.
"Despite rising vaccination rates, a return to pre-[Covid] normality seems questionable," said Ulrich Leuchtmann, head of foreign exchange and commodity research at Commerzbank.