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Two rate ratios indicating the disappearance of infections and the growth of tumours, respectively, were simultaneously plotted against the calendar years of occurrence in a period during which mortality rates were reasonably comparable to incidences. The transformation used gave upward trend time variations for infectious diseases, providing strong evidence that in Italy during the first half of this century variations in infectious diseases preceded variations in cancer. While some bacteria and viruses are known to be cancer agents, sparse studies indicate that a host's immune response to infection may destroy cancer cells. With a decreasing mortality from infectious illnesses, there may have been a reduction in the activation of immunological mechanisms against transformed cells in early phases of carcinogenesis. If cancer growth is a consequence of a lower exposure to chronic sublethal doses of microbial agents, bacterial derivates could be potentially useful in cancer chemoprevention.