Substances that can destroy or control the spread of bacteria.



Complex chemicals produced by the body's immune system to destroy invading germs or make them harmless.



A major blood vessel that carries blood away from the heart.



Very small, single-celled plants that exist in large numbers almost everywhere. Most of those that cause disease release poisonous substances into the blood which make us feel ill.



One thousand million, or 1,000,000,000.



A substance that is essential, among things, for making bones and teeth. An adult human skeleton contains about 24 pounds of calcium.



A common and dangerous disease of which there are about 150 different kinds. Cancer begins when a group of cells somewhere in the body begins to grow and multiply uncontrollably.


carbon dioxide

A gas that is taken in by plants and breathed out by mammals and other animals.



A gradual clouding of the normally clear lens of the eye. It is fairly common in old age.



The smallest living part of an animal or plant. Human beings have about 100 trillion cells, each performing a certain task.



A substance, found throughout the body, which helps to give skin strength and shape. It consists of whitish fibers bound together in bundles.



The scientific name for the way proteins, when acted on by other chemicals, can become tangled and stuck together. Because of cross-linking, the proteins may no longer work as effectively in the human body.



A disease in which the body cannot convert sugar into a form that allows it to be stored. Some patients must inject or swallow chemicals to lower the level of sugar in their blood.



Someone who gives an organ from his or her body to another person.



A long, narrow tube that a doctor can pass into a patient's body. A light, miniature TV camera, and tiny surgical instruments fitted to the end, allow observations or minor observations to be carried out by remote control.


free radical

A chemical that will easily combine with another chemical and change its character. Free radicals in the body may play a part in aging by damaging substances that are important to our health.



Each cell in a person's body contains an identical set of several thousand genes. Each gene is a complex chemical that contains instructions telling the cell what to do.



A small, living particle, usually consisting of no more than a single cell. Some germs do not affect us at all, others are important to our bodies, and still others may cause disease. Bacteria and viruses are two different types of germs.



A condition caused by unusually high pressure of the fluid in the eyeball.


heart disease

The leading cause of early death in the United States and most of the Western world. There are many forms of heart disease, but the most common involve blocked arteries.


helper T cell

A type of white blood cell that helps identify an invading germ and causes the production of antibodies and killer T cells.



The practice or study of cleanliness.


immune system

In humans, the system by which white blood cells are organized to defend the body against infection. When the immune system breaks down, a person has no defense against infection.



The process of protecting a person from a certain virus. It is done by injecting a small, treated sample of the virus so that the person's body makes antibodies against it without catching the disease.


killer T cells

A type of white blood cell that destroys body cells that have become infected by germs.



A type of white blood cell that surrounds and absorbs invading particles in the bloodstream. If it encounters a germ, a macrophage may also send out a chemical signal to attract helper T cells.



A kind of warm-blooded animal, the female of which feeds its young with milk.



A coloring chemical, or pigment, found in our hair, skin, and eyes.



A group of tissues that is organized to perform a specific function. Examples include the brain, heart, and stomach.


organ system

A group of organs that is organized to perform a specific range of tasks. Examples include the nervous system and the immune system.



A loss of minerals in the skeleton, causing bones to develop hollow spaces and become weak. Osteoporosis is particularly common in older women.



A colorless, odorless gas, making up about one-fifth of the air we breathe. Oxygen is needed by all cells to live.



Complex chemicals that play an important part in almost every body structure and process.



A person who received an organ from someone else.



A kind of cold-blooded animal with a scaly skin. Tortoises, crocodiles, and snakes are reptiles.



Animals of a certain species may have many features in common. Most importantly, males and females of the same species can successfully breed with one another to produce young.



A group of similar cells that is organized to serve a common purpose. For example, muscles are made up of tissues.



An operation involving the replacement of a faulty organ or other body part by a healthy one. The new organ may come from a donor or be artificial.



One million million, or 1,000,000,000,000.



A treated form of virus that can be given to people so that they develop immunity to the disease without actually catching it.



The smallest type of germ. Viruses can cause a wide range of illnesses, from the common cold to deadly diseases such as AIDS and rabies.


white blood cell

A cell, of which there are various kinds, produced by the body to help defend it from disease-causing germs and other invading substances.