The ancient Nubians inhabited an area in which typhus occurs, yet surprisingly few of their skeletons show the usual evidence of this disease. The skeletons do show deposits of tetracycline, an antibiotic produced by a bacterium common in Nubian soil. This bacterium can flourish on the dried grain used for making two staples of the Nubian diet, beer and bread. Thus, tetracycline in their food probably explains the low incidence of typhus among ancient Nubians. Which of the following is an assumption on which the argument relies? ['Typhus is generally fatal', 'Typhus cannot be transmitted by ingesting bread or beer contaminated with the infectious agents of this disease', 'Infectious diseases other than typhus to which the ancient Nubians were exposed are unaffected by tetracycline', 'Tetracycline is not rendered ineffective as an antibiotic by exposure to the processes involved in making bread and beer']