Hard one

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Across
  1. 4. A physical or mental feature that indicates a condition or disease, like those seen in measles.
  2. 5. A sudden occurrence of a disease in a community, such as a measles outbreak.
  3. 6. A protein produced by the body's immune system when it detects harmful substances, like the measles virus.
Down
  1. 1. A widespread occurrence of an infectious disease in a community at a particular time.
  2. 2. Separating sick individuals to prevent the spread of disease, such as those with measles.
  3. 3. The act of introducing a vaccine into the body to produce immunity to a specific disease.