ABSTRACT
Every day we eat food, breathe air, walk, talk and perform the tasks of daily life before going to bed unscathed. Some days, however, we may encounter and be harmed by viruses, bacteria, unusual antigens, extremes of temperature, chemical pollutants or fast-moving vehicles. We are harmed by environmental (or extrinsic) factors. The cause is usually obvious if it happens immediately but may be hard to discern when the effect takes days or years to manifest, e.g. tobacco smoking causing lung cancer. In some situations, the environment plays no part and the cause is entirely genetic (intrinsic), e.g. the anaemias related to abnormal haemoglobin (see page 279).
