Unit 3: Understanding Illness.

Background Bonus

Bacteria are microscopic, one-celled organisms. (Thirty trillion bacteria weigh about an ounce.) About 2,000 species of bacteria have been identified. Many of these species are beneficial to human life. Some help produce plant nutrients. Others are used in the production of cheeses, the curing of leather, and in sewage treatment. A few types of bacteria are harmful to humans.

Since the identification of these harmful bacteria, many strides have been made in protecting people from them. A century ago, at least 25 percent of American children died of bacterial infections before reaching puberty. Now fewer than 5 percent die of these infections. Pasteurization of milk and widespread use of antiseptics are helping in the battle against these germs.

Medical science has also made great strides in protecting people from serious and deadly diseases. Vaccinations, pioneered by Edward Jenner in 1798 and Louis Pasteur in 1885, introduce bacteria into the human body to help it develop immunity to certain diseases. The development of antibiotics has also helped reduce the danger of bacterial infections. In 1928 Alexander Fleming produced penicillin, the first antibiotic that could destroy disease-causing bacteria without interfering with the body's own disease-fighting system.

Unit Objectives

Students learn to

* recognize how germs can cause disease;