Emory University and University of Georgia researchers, led by Emory researcher Jacobus de Roode, studied monarch butterflies infected with the parasite Ophryocystis elektroscirrha. They said they observed higher levels of replication within the host resulted in both higher virulence and greater transmission of the parasite.

"A fundamental evolutionary question is why parasites that depend on their hosts for their own survival and fitness hurt or even kill them," said de Roode. "According to theory, parasites face a trade-off between the benefits of increased replication, the transmission to new hosts and the costs of host mortality, resulting in the highest fitness at intermediate parasite replication.

"During the past 30 years there has been very little experimental evidence that this trade-off actually exists," he added. "This is one of the first demonstrations that really shows that this trade-off model applies. These findings support the idea that selection for parasite transmission can favor parasite genotypes that cause substantial harm."

The study that included Andrew Yates and Sonia Altizer appears in the early online edition of the Proceedings of the National Academy of Sciences.