Melbourne

All very well said. The thing I would add is that contamination is somewhat "dose dependent" and you don't need to get rid of 100% of the bacteria for it to be safe to eat. You just need to reduce the amount to what your body is able to deal with (which will vary from person to person)

The biggest risk is situations that allow bacteria to breed - so using one set of tongs to move a steak into the pan, then using the same pair to transfer the cooked meat to the plate is quite low risk because the amount of bacteria that could have been transferred to the tongs and back to the steak is pretty tiny and would not be enough to cause harm. However if you were planning on cooling the same steak to cut up and use in a salad the next day it becomes riskier, because that tiny amount of bacteria is being time and opportunity to grow before you eat it. Similarly using the marinade your meat has been in as a baste at the end of cooking is risky because the marinade is an environment that allows bacteria to breen and build up to harmful amounts.

The time issue is also a reason why some things can be safe for a home cook but unsafe in a commercial environment - large batches of food take longer to heat and cool, and are often kept waiting longer to be served, so they are in the "danger zone" where bacteria can breed and increase for a longer time.