Expert: Florence M Rollwagen - 5/21/2008

Question
what would provide the best evidence that a particular virus caused a certain disease?

Answer
Hi Alyssa:  Thanks for your question.

When we attempt to identify the causative agent for a particular disease, we use a set of rules called "Koch's Postulates"  Koch was a 19th century microbiologist who tried to determine the infectious nature of many common diseases such as cholera and tuberculosis.

The four postulates are:

1.  The microorganism must be found in abundance in all organisms suffering from the disease, but not in healthy organisms.

2.  The microorganism must be isolated from a diseased organism and grown in pure culture.

3.  The cultured microorganism should cause disease when introduced into a healthy organism.

4.  The microorganism must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

There are exceptions to Koch's postulates for a number of reasons:

It is not possible to grow the organism in pure culture.  Heptatitis viruse falls into this category.

Organisms that grow only in humans (leprosy, syphilis, HIV) cannot be re-inoculated into another human to test postulate #3 because of ethical considerations.  This exception frequently can be met by examining patients infected by accidental needle stick, such as doctors and nurses.

I hope this answer has helped you.  Please write back if you have more questions.

FM Rollwagen, PhD