Yet, there is ample reason to suspect that there are strains of HIV that differ in degree of virulence, and that the disease course of individuals infected with them will progress relatively faster or slower as a result. And, in a now famous instance involving the Sydney Blood Bank Cohort, an individual infected with a genetically crippled strain of HIV passed his virus on to 6 others via blood transfusion, and in all recipients, the rate of disease progression has been extremely slow or nonexistent. The answer is, there isn't. But what can be measured is the relative fitness of strains of HIV obtained from infected individuals. Thus, for HIV, the most fit strains are those that propagate fastest in cells and produce the greatest numbers of progeny.
Fitness is an approximation of virulence.
The relative growth rate of individual isolates of HIV, their fitness, can be evaluated in tissue culture. The speed with which the virus spreads among human cells in a culture dish and the numbers of progeny produced can be measured directly. Accordingly, it should be possible to evaluate the relative fitness of strains of HIV by individually propagating them in culture and comparing the growth rates observed. This method provides an accurate way to assess relative fitness. Cultures of human white blood cells are inoculated simultaneously with both viruses, thereby allowing the 2 strains to compete with one another.