"We’re all too familiar with the viruses that can infect us, from the common cold to yellow fever virus to the endogenous retroviruses that make up a chunk of our genome. Many of us are also acquainted with parasites, such as tape worms or Giardia, that like to set up camp in the human body. But the world of parasites and viruses does not end there. Many parasites or endosymbionts can be infected with viruses. A classic example is Paramecium, which can harbor an endosymbiotic bacterium, Caedibacter, which in turn carries phages involved in making a toxin. But from the human point of view, things start to get particularly interesting when we consider the viruses that infect parasites of humans and how those viral infections—inside of a parasite inside of a person, somewhat like a Matryoshka nesting doll—may modulate the parasite’s interaction with its human host."
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