The Ma­tryoshka Dolls of Hu­man Pathogens by Jamie Schafer — 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…