The topic you're wondering about is called planetary protection and it is taken very seriously. All spacefairing nations are signatories to the Outer Space Treaty where this first got formalized.

The required planetary protection mitigations depend on where you are going and what you are doing. For instance, there is no evidence that Mercury has/had life and it isn't interesting with respect to biological proceses so you don't have to do anything. But for a life detection to an interesting place, like the Mars Viking landers, there is a lot you need to do. For those missions, the entire spacecraft was put into a bag and baked. At the end of the process it was estimated that there were just 30 viable bacterial endospores left on it. Endospores are the most hardy of the bacteria so that means everything else was killed and you have a nearly sterile spacecraft. You'd have to do the same type of thing for a mission to Europa because it could harbor life.

MSL didn't have to do as much because it isn't a life detection mission, nor is it going to a place on Mars that is likely to have present life (because there's no water source nearby).