They tried introducing a sexually-transmitted disease of plants, called "anther smut," into hosts that were already infected, and found that strains are more likely to share a host if they are more closely related.

In this way "pathogens seem to interact as we would," says Koskella, "sharing resources with close relatives and being more competitive towards non-relatives."

According to current evolutionary theory, the competition among pathogen strains determines why some diseases cause very severe symptoms, "so it is crucial to understand when and how this competition occurs within the hosts," adds Koskella.

To win host resources away from a non-relative, theory assumes the pathogens will increase exploitation of the host and thus cause more severe symptoms. However, if pathogens share the host most often with close relatives, this evolutionary conflict is decreased, possibly resulting in less harm by overexploitation. Such new insights into how pathogens interact within the host are needed to better understand and confront the harmful effects of diseases.

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