The markets are not particularly consequential at the moment. Voting has already begun, and I doubt the American public will be voting based on stock market peformance.
However, there are many of his Republican colleagues that have disavowed his comments (or lack of comments) regarding his stance on White Supremacy in the last debate. Guess what? He now has to quarantine for 14 days - at least. That would make him ineligible for the next Presidential debate.
Major news all over the world was solely focused on his peformance in the debate, and now the entire conversation has been shifted to him contracting covid.