Disease is a condition that disrupts the normal, healthy functioning of a living thing's body. When you catch a cold, develop strep throat, or get the flu, you have a disease. Your body isn't working the way it should: you might have a fever, feel exhausted, or struggle to breathe normally.

Diseases come from different sources. Some are caused by germs like bacteria or viruses that invade your body. Others develop when something goes wrong inside your body, like when someone's pancreas stops making enough insulin and they develop diabetes. Some diseases, like certain allergies or heart conditions, can run in families.

Being diseased means you're uncomfortable, unwell, or unable to do normal activities easily. A diseased tree might have rotting bark and dying branches. A diseased lung doesn't work properly.

Scientists and doctors who study diseases have learned to prevent, treat, or cure many of them. Vaccines stop certain diseases before they start. Antibiotics fight bacterial diseases. Even though some diseases remain difficult to treat, medical researchers continue working to understand how diseases develop and how to stop them. The discovery that germs can cause disease in the 1800s revolutionized medicine and saved countless lives.