per WebMD --
"In the case of SARS, we know that the coronavirus had been found and grown from several individuals who have been sick with the symptoms of SARS -- thus fulfilling the first two of Koch's postulates. Because it would be unethical to expose people with the virus, public health scientists use a science called epidemiology to prove that only people exposed to the virus have gotten the infection. This technique relies on interviewing and studying groups of people who have gotten ill and comparing them with people who have not come down with the disease. Investigators then assume the disease would occur if a person were exposed to the disease."
((link -- https://www.webmd.com/a-to-z-guides/how-scientists-identify-virus ))
"In the case of SARS, we know that the coronavirus had been found and grown from several individuals who have been sick with the symptoms of SARS -- thus fulfilling the first two of Koch's postulates. Because it would be unethical to expose people with the virus, public health scientists use a science called epidemiology to prove that only people exposed to the virus have gotten the infection. This technique relies on interviewing and studying groups of people who have gotten ill and comparing them with people who have not come down with the disease. Investigators then assume the disease would occur if a person were exposed to the disease."
((link -- https://www.webmd.com/a-to-z-guides/how-scientists-identify-virus ))
Either way, the technique doesn't fulfil Koch's postulates, as it is not demonstrated that what is alleged to be the virus causes sickness. It could be that the alleged "coronavirus" identified is part of the body's immune reaction to sickness, therefore present in people exposed to it.