People, for example, are often infected by tiny bacteria, but bacteria, in turn, can be infected by even more minute viruses. Hundreds of different diseases exists. Each has its own particular set of symptoms and signs, clues that enable a physician to diagnose the problem. A symptom is something a patient can detect, such as fever, bleeding, or pain. A sign is something a doctor can detect, such as a swollen blood vessel or an enlarged internal body organ.
Every disease has a cause, although the causes of some remain to be discovered.
Every disease also displays a cycle of onset, or beginning, course, or time span of affliction, and end, when it disappears or it partially disables or kills its victim. An epidemic disease is one that strikes many persons in a community. When it strikes the same region year after year it is an endemic disease. An acute disease has a quick onset and runs a short course. An acute heart attack, for example, often hits without warning and can be quickly fatal.