An unlikely hero

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Across
  1. 5. the hard upper shell of a turtle or a crab
  2. 7. a group of atoms bonded together
  3. 8. extremely serious
  4. 9. pollute
  5. 12. a substance used to stimulate antibodies to provide immunity
  6. 13. animals that have a backbone
Down
  1. 1. ancient
  2. 2. relating to medicines or their preparation
  3. 3. system a group of network of cells that defend the body against infection
  4. 4. an outbreak that is widespread across a whole country or the world
  5. 6. a period of ten years
  6. 10. Any cell capable of moving
  7. 11. man made