Technology in medicine has developed greatly since the 1800s and now allows doctors the ability to make an accurate diagnosis with minimal invasion or trauma to the patient.
For example, let's take a simple torn meniscus in the knee. In the 1800s there was no such thing as an MRI machine, so diagnosing the pain caused by that cartilage tear would have involved joint manipulation and that's about it. Modern day doctors will still do this to rule out the difference between a meniscus tear and an ACL tear, but once ruled a possible tear, the doctor can order an MRI scan of the knee (he could do this without the physical exam, but that's not typically done). After the scan, the MRI will confirm the tear, show the doctor the location of the tear, and the severity. That means the doctor can make an accurate judgment about the success of surgery and then know exactly where to go in the surgery. Before, the doctor would have had to open the knee up completely in order to see what was happening.
If you don't want injury tech, then technology has allowed a vast amount of information to be obtained from a simple blood test. What's the red cell count? What's the white count? Are there platelets present? In the 1800s it was common to do "blood letting" if a person was sick, which drains the person of the very fluid that is able to fight off any illness.
The simple diagnostic testing of today allows doctors to see and fix potential hazards before they ever have a chance to become big medical problems.