The novel coronavirus and the SARS outbreak of 2003 have two things in common: Both are from the coronavirus family, and both most likely started in wet markets.

At such markets, outdoor stalls are squeezed together to form narrow lanes, where locals and visitors shop for cuts of meat and ripe produce. A stall selling caged chickens may abut a butcher counter, where meat is chopped as nearby dogs watch hungrily. Some vendors hock hares, while seafood stalls display glistening fish and shrimp.

Wet markets put people and live and dead animals — dogs, chickens, pigs, snakes, civets, and more — in constant close contact. That makes it easy for zoonotic diseases to jump from animals to humans.

“Poorly regulated, live-animal markets mixed with illegal wildlife trade offer a unique opportunity for viruses to spill over from wildlife hosts into the human population,” the Wildlife Conservation Society said in a statement.

In the case of SARS and the new coronavirus disease, called COVID-19, bats were the original hosts. The bats then infected other animals, which transmitted the disease to humans. The coronavirus has now killed at least 2,700 people and infected more than 80,000 others.