In otherwise healthy people, a heart rate at rest should be less than 100 beats per minute at rest. Heart rates that are consistently above 100, even when the person is sitting quietly, can sometimes be caused by an abnormal heart rhythm. A high heart rate can also mean the heart muscle is weakened by a virus or some other problem that forces it to beat more often to pump enough blood to the rest of the body.
Usually, though, a fast heartbeat is not due to heart disease, because a wide variety of noncardiac factors can speed the heart rate. These include fever, a low red blood cell count (anemia), an overactive thyroid, or overuse of caffeine or stimulants like some over-the-counter decongestants. The list goes on and includes anxiety and poor physical conditioning.