Why is it so difficult to diagnose an infectious disease on the spot?

We measure vital signs such as temperature or blood pressure routinely, yet we have no quick way to pinpoint the cause of most infection. Our inability to identify harmful bacteria and viruses costs patients dearly. During the several days or so that doctors usually take to identify bacteria and viruses, illnesses spread and may get harder to treat, and the most vulnerable patients—newborns, the elderly, anyone with a weak immune system—can die.