Bacterial infections, such as those caused by certain strains of E. coli, can be transmitted from animals to humans through contaminated food. Although the outbreak of E. coli infections in North America in the early 2000s likely originated from specific livestock in the Midwest, the farmers who raised these animals did not report any illness. Therefore, it is reasonable to conclude that these farmers had some form of immunity to the E. coli strains present in their livestock.

Which of the following, if true, most seriously weakens the argument?