The major causes of cancer fall into two categories: those over which we have some control and the rest. The latter includes radiation from the Earth that has been a background to human evolution and about which we can do nothing, although we could take more effective steps to limit the accumulation of radon in houses in regions where there is high, localised emission of the gas. The factors that we can control also fall into two groups: those exerting major effects for which the epidemiological evidence is overwhelming and those for which the data are inconclusive and therefore controversial. Of the former, the most familiar is the use of tobacco, which is responsible for 90% of lung tumours. By contrast, there are a number of prominent agents that may have weak tumour-promoting effects. For these, the epidemiology is generally unpersuasive and direct experimental evidence has not been forthcoming, as exemplified by the continuing debate over the risks associated with the use of mobile phones. The two most effective measures we could take that would reduce the global cancer burden by at least one third would be to abolish the use of tobacco and to limit the consumption of red meat.
In fact, nearly 20% of all cancers worldwide are caused by infection, either by viruses, bacteria or other microorganisms. In developing countries, this figure rises to about one in four. While viral transmission between individuals is a major factor, living conditions and especially sanitation also bear on cancer, partly because the chronic infection is likely to decrease the efficiency of the immune system. Quite how the immune system targets tumour cells at the molecular level remains controversial but it is unquestionably one of our anti-cancer defences and its suppression can release one of the brakes on tumour development.
The impact of microbial infection is most clearly established for stomach cancer in response to Helicobacter pylori, the bacterium mentioned earlier that gives rise to chronic infection and ulcers in the stomach. It is mainly spread by the consumption of food or water contaminated with faecal matter and it can double the risk of developing gastric cancer although the most potent strains may increase the risk by 30-fold. About a quarter of us pick up H. pylori and there are effective drug combinations for treating this infection. Given that most of those infected show no symptoms and stomach cancer is an uncommon outcome, we’re clearly quite good at keeping it under control ourselves.
Another bug, tuberculosis (TB) bacterium (Mycobacterium tuberculosis), not only kills two million people a year but is present in latent form in about one in three of us. Effective drug therapy is available and the mortality rate is less than 5%. It’s been known for many years that patients with TB have an increased frequency of lung cancer, possibly as a result of inflammation, and recent evidence from mice indicates that chronic infection can indeed initiate the development of malignant lung cancer.
Although, as we shall see, cancers essentially arise from genetic defects, a microbial or viral infection can act as an initiating event.
Everything that we see in the world around us comes, quite literally, in the form of electromagnetic radiation, an oscillating electric and magnetic field that travels at the speed of light (Fig. 1). The electromagnetic spectrum is the range of the frequencies (or wavelengths) of electromagnetic radiation. This includes a tiny region that our eyes can detect (the visible spectrum) as well as radio waves and X-rays. At shorter wavelengths is ‘ionising radiation’, which includes gamma rays, sub-atomic particle interactions (e.g. in radioactive decay), and X-rays that result from high-speed electrons colliding with metal. Alpha and beta particles are also forms of ionising radiation released when unstable isotopes undergo radioactive decay. Alpha particles are made up of two protons and two neutrons; beta particles are electrons emitted by nuclei.
1. The electromagnetic spectrum.