I have to vigourously disagree with this blanket statement. There are just too many variables of situation and EMS availability to follow a flat rule. Leaving it to the "professionals" too often is a way to let someone die or suffer long term unnecessary injury.

A better thing is to become well enough trained and informed so you can make a good decision. It is a sad thing indeed to see a bunch of people standing around watching someone die while waiting for the "professionals" just because they are afraid of doing something. The answer is to become educated. After all the person you save may be a loved one.

People are way too scared of infection from a random diseased person. Even if you get body fluids on you from such a person prompt cleaning negates much of the risk. If you cannot overcome your fear then carry a pair of gloves in your pocket.

In short, don't let fear overcome duty to other humans. Would you want to be left to die or have life long injury just because someone considered you too dirty to help, or because bystanders were too afraid?

Get trained and educated. The life you save may be your own or a loved one.