| 1. | The microorganism must be detectable in the infected host at every stage of the disease. |
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| 2. | The microorganism must be isolated from the diseased host and grown in pure culture. |
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| 3. | When susceptible, healthy animals are infected with pathogens from the pure culture, the specific symptoms of the disease must occur. |
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| 4. | The microorganism must be re-isolated from the diseased animal and correspond to the original microorganism in pure culture. |