How can a virus be determined to cause a disease?
A modified form of Koch's postulates is used to guide experiments that establish that a particular virus causes disease.

As a specific example of such a study, the SARS-CoV virus was determined to cause the SARS disease by fulfilling these criteria:


SARS virus is found in people suffering from SARS, and not found in those who are healthy.
The virus can be isolated from infected people and grown in cell cultures (here, Vero cells).
Viruses grown in culture can be reisolated and compared with (and found identical to) the suspected pathogenic virus.
This isolated virus can re-infect people (or infect model organisms; in this case, a species of macaques).
Virus in reinfected organisms can be isolated, grown in culture, and found identical to the original pathogen.
The immune system is observed to respond in similar ways to the infection (pneumonia and lung tissue lesions).