ABSTRACT
Cancer is not a single disease, but a collection of diseases related by the fact that the affected genes change the ways in which cells respond to various signals that normally control cell division, growth, and response to surrounding cells and tissues. Cancers vary in their incidence and virulence. For example, while breast cancer has the highest rate of incidence (Figure 18.1), lung cancer has the greatest mortality rate (Figure 18.2). Cancer can occur in virtually any multicellular organism, and in some ways can be conceptualized as an evolutionary disease. That is, the same mechanisms responsible for evolution, such as mutation, recombination, gene duplication, gene amplification, chromosomal translocations, and other phenomena, also can lead to initiation of a cancer cell. Because cancer cells begin as normal cells, they are not recognized as foreign or dangerous cells by the immune system of the organism. The cells proliferate to form small foci, and then larger tumors, many of which attract blood supplies, and then migrate to other parts of the organism to set up colonies in other locations. This often makes treatments and cures difficult.
