"All of the evidence points to this not being a laboratory accident," says Jonna Mazet, a professor of epidemiology at the University of California, Davis and d

"All of the evidence points to this not being a laboratory accident," says Jonna Mazet, a professor of epidemiology at the University of California, Davis and director of a global project to watch for emerging viruses called PREDICT. Rather, the experts interviewed by NPR all believe that the virus was transmitted between animals and humans in nature, as has happened in previous outbreaks — from Ebola to the Marburg virus — and with other known coronaviruses such as SARS and MERS.