The most common cause of disease is infection by microorganisms that find their way into internal body tissues, where they multiply and disrupt normal cell function. These organisms, which are commonly known as germs, take a wide range of forms and contain the groups classified broadly as viruses, bacteria, protozoa, and fungi. Disease can also be caused by larger, more complex organisms, such as parasitic worms and their larvae, which may infest various parts of the body, especially the intestine.
The likelihood of serious illness from infectious disease is largely influenced by the environment. In the developed world, most infections can be treated effectively or prevented. However, in the developing world, where children are often malnourished, many of the common infections, such as measles, can be fatal. People living in temperate regions are rarely affected by cholera, malaria, or parasitic worms, but in the tropics such disorders are frequent causes of ill health and death.
Sometimes, an infectious disease spreads rapidly worldwide. Influenza, for example, tends to occur in annual outbreaks in all countries. Among the more recent global health threats are newer diseases, such as AIDS, as well as long-established ones, such as tuberculosis, in which organisms have become resistant to common drugs.
The organisms that cause disease enter the body in a variety of ways. Some are breathed in or swallowed in food and water; others may gain entry through a break in the skin or be transmitted during sexual contact. An infection may spread throughout the body, affecting several organs at once. However, certain infectious organisms target and damage only one particular organ or part of the body, such as the liver, respiratory tract, or intestine.
Infections are more likely to develop if the number of infecting organisms is large or if a person’s resistance to disease is reduced. Reduced resistance may be due to factors such as extremes of age, poor nutrition, or an immune system already weakened by disease.
Over the last 100 years, great advances have been made in the control of infections, largely due to improvements in diet, housing, and hygiene. In addition, the increasing availability of routine immunizations and drugs such as antibiotics have made it possible to cure or even wipe out many infectious diseases. International programmes to monitor the occurrence of infections have also helped to check the spread of many often fatal diseases.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.