Every day your body wages war against enemies that are silent and unseen but potentially deadly. Foreign invaders, such as bacteria, viruses, and parasites, threaten your health. a You are not likely to be aware of those battles because your immune system repels or destroys most of the invaders before the onset of symptoms. Sometimes, however, the harmful germs gain the upper hand. If so, you may need to bolster your defenses with medicine and other treatments.

For thousands of years, people knew virtually nothing about the dangers of microscopic or other small harmful organisms. However, when 19th-century scientists confirmed the link between germs and disease, we became better equipped to defend ourselves. Medical researchers have since eliminated or greatly reduced the threat of some infectious diseases, including smallpox and polio. Recently, however, others, such as yellow fever and dengue, have made a comeback. Why? Consider these factors:

• Every year, millions of people travel around the globe, often transporting disease-causing agents. According to an article in the journal Clinical Infectious Diseases, “virtually all of the contagious virulent infections” can be spread by international travelers.

• Some bacteria have developed resistance to antibiotics. “The world is heading towards a post-antibiotic era, in which common infections . . . can once again kill,” states the World Health Organization.

• Civil unrest and poverty often hinder government efforts to control the spread of disease.

• Many people lack practical knowledge of how to prevent disease.

Despite these disturbing trends, there is much you can do to protect yourself and your family. The following will show that, even if you live in a developing land, simple and effective strategies may be within your reach.