Why do we fall ill?

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Across
  1. 4. an infectious agent that can only replicate within a host organism.
  2. 5. The invasion and growth of germs in the body
Down
  1. 1. resistant to a particular infection or toxin owing to the presence of specific antibodies or sensitized white blood cells.
  2. 2. conditions or practices conducive to maintaining health and preventing disease, especially through cleanliness.
  3. 3. the state of being free from illness or injury.