I am afraid that those comments go back to the late 80's. At that time I was a skeptic — the argument based on Koch's postulates to try to distinguish between cause and association. … Today I would regard the success of the many antiviral agents which lower the virus titers (to be expected) and also resolve the failure of the immune system (only expected if the virus is the cause of the failure) as a reasonable proof of the causation argument

Personal communication (4 April 2006) quoted in "Well, someone has to do it" at Moment of Science (July 2006)