The future contracts for both the S&P 500 and the Dow industrials lost 1.9 per cent.
Oil prices also slipped.
Trump tweeted news of his test results just hours after the White House announced that senior aide Hope Hicks had come down with the virus after travelling with the president several times this week.
To say this potentially could be a big deal is an understatement, Rabobank said in a commentary.
Anyway, everything now takes a backseat to the latest incredible twist in this US election campaign.
Trading in Asia was thin, with markets in Shanghai and Hong Kong closed. The Nikkei 225 index shed strong early gains, losing 0.8 per cent to 22,999.75 after the Tokyo Stock Exchange resumed trading following an all day outage due to a technical failure.
Reports that the Japanese government is preparing new stimulus measures to help the economy recover from a prolonged downturn worsened by the coronavirus pandemic provided only a temporary lift. Prices fell further after Trump's announcement.
Australia's benchmark S&P/ASX 200 slipped 1 per cent to 5,815.90. Shares in Singapore, Thailand and Indonesia also fell.
On Thursday, the benchmark S&P 500 ended the day 0.5 per cent higher, at 3,380.80, the Dow Jones Industrial Average rose 0.1 per cent to 27,816.90 and the Nasdaq composite rose 1.4 per cent to 11,326.51, as big tech-oriented stocks propped up the market, much as they have through the pandemic.
(Only the headline and picture of this report may have been reworked by the Business Standard staff; the rest of the content is auto-generated from a syndicated feed.)