We are not alone. No matter how clean we are or how healthy we feel, we carry around on our bodies billions of microbes—microscopic one-celled organisms called bacteria and viruses.
Agnolo di Tura chronicled this tragic yet common occurrence in Siena, Italy, in 1348. Called the Great Pestilence, the disease was later known as the Black Death or bubonic plague, and it swept through Europe with frightening speed.
Humans have evolved along with bacteria and viruses for millions of years, so it should be no surprise that the human body has developed a system of keeping the harmful microbes out. The first line of defense is the skin, which is a sheath of closely interlocking cells.
Many infectious diseases, like smallpox, polio, and anthrax, are ancient and have plagued humankind for thousands of years. But new strains of bacteria and viruses continue to emerge seemingly out of nowhere to cause mysterious new ailments.
For centuries people have been using microbes to their advantage, turning grapes into wine, milk into cheese, and cabbage into sauerkraut. People benefit from what microbes do naturally: They eat.
A bacterium was the birthplace of genetic engineering, one of the most revolutionary technologies in science. And since those first experiments with bacteria back in the 1960s, hundreds of new medicines and products have become commonplace.