Robert Koch, studying the causes of anthrax sheep disease, identified a pure culture of microorganisms taken from a sick sheep. He introduced this culture to 70 other healthy sheep, of which 50 fell ill with anthrax and died. Answer the following questions: what is the cause of the death of 50 sheep; why did 20 sheep survive, is the culture isolated by Koch a vaccine? Explain the answer.
1) Sheep died because they were infected with anthrax.
2) 20 sheep had natural immunity against this disease (either: they had been ill earlier and acquired immunity to this disease).
3) The culture isolated by Koch is not a vaccine, as it is not weakened, and the surviving sheep were not previously introduced
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