Stock markets plunged around Asia on Monday, as panic selling set in with traders fretting over the economic impact of the new coronavirus and digesting a free-fall in the oil price.By mid-morning, the benchmark Nikkei 225 index had dropped 5.10 percent or 1,058.06 points to 19,691.69, while the broader Topix index was off 5.01 percent or 73.69 points to 1,397.77.