After sufficient scientific study led to the development of germ theory, pasteurization was introduced in the United States in the 1890s. This move successfully controlled the spread of highly contagious bacterial diseases including "E. coli", bovine tuberculosis and brucellosis (all thought to be easily transmitted to humans through the drinking of raw milk). In the early days after the scientific discovery of bacteria, there was no product testing to determine whether a farmer's milk was safe or infected, so all milk was treated as potentially contagious. After the first tests were developed, some farmers took steps to prevent their infected animals from being killed and removed from food production, sometimes even falsifying test results to make their animals appear free of infection. Recent advances in the analysis of milk-borne diseases have enabled scientists to track the DNA of the infectious bacteria to the cows on the farms that supplied the raw milk.