Disease is a condition that affects how the body works. It can change how a person feels or functions. Diseases can have different causes.

Some are caused by bacteria or viruses, such as influenza or COVID-19. Others, like diabetes, are linked to how the body processes energy. Diseases can affect humans, animals, and plants. Some spread between people, while others do not.

Doctors and scientists study diseases in medicine and biology. Information is collected through hospitals, laboratories, and global health systems.