Anthrax is a fatal, contagious disease that affects farm animals and humans. This disease has received a lot of media attention because it is one of several that can be used as a biological warfare agent. To learn more about this disease, scientists at the biological warfare research center at Fort Dietrich, Maryland set up the following experiment. One herd of 100 cows was injected with an experimental vaccine composed of a liquid containing anthrax bacteria that has been weakened by heating. A second herd of 100 cows was injected with the same liquid, minus the heat treated bacteria. A few weeks later, the scientists injected both herds of 100 cows with full strength bacteria. The cattle that had been previously injected with weakened bacteria showed no symptoms of the disease. The cattle from the second herd became ill and died.
what is the control group for this experiment