The virulence of a virus,perceived by the speading speed of the virus within a population,reflects the population' s genetic heterogeneity.When the difference of morbidity and mortality of a virus-caused disease within various ethnic groups is trivial,the virulence of the virus may be very powerful.In contrast,when the difference is significant, the virulence is quite low.The greater the difference between the ethnic groups,the lower the virulence.