A study of the wildlife trade in three provinces in southern Vietnam produced startlingly clear confirmation for one of the underlying objections to the wildlife trade in Asia — the trading offers an ideal opportunity for viruses in one animal to infect another.

In field rats, a highly popular animal to eat in Vietnam and neighboring countries, the percentage that tested positive for at least one of six different coronaviruses jumped significantly. It increased from 20 percent of wild-caught rats sold by traders, to slightly more than 30 percent at large markets, the next step in the supply chain, to 55 percent of rats sold in restaurants that tested positive.

In other words, the odds were about even that a field rat on the plate was infected with some kind of coronavirus.

The data for this study has taken a long time to process. The tests were done in 2013 and 2014, for common coronaviruses, long before the emergence of the treacherous novel virus that has caused the current pandemic. The results show unequivocally how viruses spread from animal to animal as they are transported in crowded conditions to market.