An epidemic of influenza that swept the globe in 1918 killed between 20 million and 40 million people. Communicable diseases can spread from one person to another and are caused by microscopic organisms that invade the body. Noncommunicable diseases are not communicated from person to person and do not have, or are not known to involve, infectious agents. Some diseases, such as the common cold, and come on suddenly and last for no more than a few weeks. Other diseases, such as arthritis, are chronic, consistent for months or years, or reoccur frequently.
Every disease has certain characteristic effects on the body.
Some of these effects, include fever, inflammation, pain, fatigue, dizziness, nausea, and rashes, are evident to the patient. These symptoms offer important clues that help doctors and other health care professionals make a diagnosis. Many times, the symptoms point to several possible disorders. In those cases, doctors rely on medical tests, such as blood examinations and X rays, to confirm the diagnosis. Communicable diseases are caused by microscopic organisms.