Why did the Sensex crash by 806 points on 24 February? Did it have anything to do with the Chinese virus?

You are largely correct that the correction was essentially driven by the Chinese virus syndrome. Till date, nearly 2600 people have lost their lives to this pandemic and the markets are worried that the pandemic is now spreading to more and more countries. There was a virtual sell-off across global markets purely for this reason and that rubbed off on India also. There was also some disappointment on the Trump visit as the markets were expecting that there could be some indications of a free trade agreement (FTA) but not has been forthcoming on the first day of his visit. IMF has already warned that global growth may have to be once again downgraded and that also had a rub off effect on the stock markets. In addition, the 12,000 level for the Nifty was proving to be a difficult resistance. The next thing to see is if Sensex is able to hold 40,000 and if that is breached then the correction could become much sharper.