One major hypothesis is that the pandemic began with zoonotic spillover, a leap from animals to people, perhaps from bats to an intermediate host. The vast majority of human viruses have zoonotic origins. Bats harbor a large variety of viruses, and huge populations of bats roost in southeast Asia. One of the genetic sequences that is closest to SARS-CoV-2, known as RaTG13, with 96.2 percent genetic similarity, was found in 2013 in a bat cave in Yunnan province in southwestern China. Another three bat species, found recently in caves in northern Laos, yielded viruses with up to 96.8 percent similarity to the pandemic strain. Wildlife farming was quite widespread in China before the pandemic. Live, slaughtered and frozen wildlife was sold at the Huanan seafood market in Wuhan and many other markets in China. In December 2019, the Huanan market was linked to human cases, though it appears to have been an amplifier of the spreading virus, not the original source.