Often times, an illness or injury becomes more and more difficult to treat as time goes on. It is often critical that a doctor diagnose an illness or injury in an appropriate amount of time, so that treatment can be administered, and the patient made well.
A common example is a case of cancer. If cancer is diagnosed early on, then the patient stands a much greater chance at recovery than if it is allowed to spread. Diagnosis of cancer can be done early on if the doctor is given a chance to examine the patient, such as at a routine check-up, or a visit to the hospital for some potentially related symptoms. If the doctor does see the patient, and fails to diagnose potential symptoms of cancer, then the patient may be sent home and the cancer will be allowed to spread. Whether or not this is negligent treatment depends on the circumstances of the case.