How Scientists Identify a Virus

Many people wonder just how scientists know that the cause of SARS is a virus and, more importantly, this particular virus.

Public health scientists verified that a common virus -- a coronavirus -- that has become more severe as the likely cause of severe acute respiratory syndrome (SARS). Many people wonder just how scientists know that the cause is a virus and, more importantly, this particular virus.

In 1890, Robert Koch described the basis rules that scientists use to determine if an infectious organism causes a specific disease. These four rules are called "Koch's postulates."

1. The organism must be found in people with the disease and be absent in people without the disease.
2. The organism must be able to be grown from tissues or other specimens from the affected individual in the laboratory.
3. The organism must cause the disease when given to an unaffected healthy person.
4. The organism must again be grown from this second individual.

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. They then look to see if newly unintentionally exposed people come down with the disease and that organism is grown from them. This fulfills in principle Koch's third and fourth rules.

In the case of SARS, scientists have clearly shown that the virus is associated with people with the disease and the virus has been isolated from these patients.  The epidemiology also shows that the disease occurs in people who are exposed to the disease more often than people who have not been clearly exposed to the disease. Finally, the virus has been grown from the people who were subsequently exposed. In addition, scientists can use animals to demonstrate these last two rules by exposing the animal to the coronavirus and see if it causes a disease like SARS.