Choose the option that best answers the question.

The prevalence of a simian virus has been directly correlated with population density in gorillas. Recent fieldwork in the Republic of Dunaga, based on capturing gorillas and testing the gorillas for the virus, has shown that Morgania Plain gorillas are more than twice as likely to be infected than are the Koluga Mountain gorillas. Nevertheless, the population density of Koluga gorillas is significantly greater than that of Morgania gorillas.

Which of the following could best account for the discrepancy noted above?