ScienceDaily (Feb. 11, 2011) � A parasite and a virus may be teaming up in a way that increases the parasite's ability to harm humans.
When the parasite Leishmania infects a human, immune system cells known as macrophages respond. However, some Leishmania strains are infected with a virus that can trigger a severe response in macrophages, allowing the parasite to do more harm in animal infections.
According to Beverley, the results suggest that some viral infections in Leishmania may be improving the parasite's chances to infect the mammalian host's immune cells. He speculates that this increased pathogenicity may be one evolutionary trade-off that makes losing the RNAi pathway worthwhile for Leishmania and other microbes.