Need help please?

Anthrax is a fatal, contagious disease of people and darm animals. in 1881, Louis Pasteur had an idea for immunizing animals against athrax. He collected some anthrax bacteria from infected animals. He heated the bacteria to weaken, but not kill them. Pasteur injected half of a heard of sheep with a saline solution containing the weakened bacteria. The other half were inject only with saline. Both groups were then exposed to health anthrax bacteria. The sheep that had been immunized with the weakened bacteria remained healthey, but the sheep injected only with saline died. Are the sheep that remained healthy demonstrating acto or pive immunity?