How do you make a parasite less harmful? It might be as simple as forcing them to stick around.

Amanda Gibson from Indiana University performed experimental evolution with the nematode worm Caenorhabditis elegans and a parasitic bacteria (Serratia marcescens), comparing different scenarios for how the parasite was passed on. Under most circumstances, parasites continued to cause their hosts harm throughout the 20-generation experiment. Only when each strain of parasite was allowed to infect the same host strain, generation after generation, did the parasite evolve to become less harmful.

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