What is the connection between virus prevalence and animal populations?

Rinderpest disease is caused by a virus that affects hoofed animals, including cattle and wildebeest. In the 1950s, a cattle vaccination program was implemented to eradicate the disease in the Serengeti, and this led to dramatic changes in the populations of wildebeest and other species. The figure shows the number of wildebeest in the Serengeti ecosystem (shaded circles, left y-axis) and the prevalence (i. e., percentage) of individuals infected by rinderpest disease (unshaded squares and triangles, right y-axis) from 1958 to 2003.