Many people believe that there is other life out there in the vastness of the universe. In fact, statistically, it seems that life must exist elsewhere. There are some 200-400 billion stars within the Milky Way alone, and many galaxies have a lot more stars than our own (like the Andromeda Galaxy). Of course, there are also galaxies that have fewer stars (dwarf galaxies, generally, like the Large and Small Magellanic Clouds). But even if all the galaxies were smaller than the Milky Way, it's estimated that the observable universe contains somewhere between 100-200 billion galaxies, with some estimates going as high as 500 billion! You don't have to be a serious mathematician to understand just how MANY stars may be out there, and it seem that a majority of them are harboring planets.This makes it extremely likely that have has evolved on at least a few of them.
Of course, this is just probability and speculation. But let's suppose that life does exist on another world. What happens if we travel there? How might we impact this life? One unfortunate possibility is that we unintentionally eradicate the native life by introducing a harmful parasite, bacteria, or other similar threat into the ecosystem. This is a serious issue when considering exploring other worlds, even in our own solar system.
Infecting other planets with lifeforms indigenous to Earth is something that is taken very seriously by NASA and other space agencies, especially when probes are being sent to other worlds that might harbor primordial (or fossilized) bacteria or microbes (such as Mars or any of the Galilean moons [Jupiter's moons]).
Before anything is sent into space, technicians ensure the equipment is cleaned thoroughly and freed of all biological contaminants. To get a bit of an idea on how strict they are on their cleaning, have a look at this document.
Of course, the contamination prevention is two-fold. It keeps contamination out of spacecraft itself, away from things that could interfere with normal working operations, and also stops contamination from entering other places in the solar system. It also helps ensure that our findings are as accurate as possible. We certainly don't wan to discover microbial life on Mars, only to discover that it hitched a ride on Curiosity.
Moreover, even if a satellite or probe isn't going anywhere near a planet or moon, there is nothing to stop bits of Earth from migrating out into the cosmos if an accident occurred (like a collision with even a small asteroid).
So let's say that, tomorrow, Curiosity finds indisputable evidence of past or present life on Mars. First, this is something that would obviously be one of the most monumental finds in human history. However, not all contamination comes from man. It's very likely that bits of this planet are laying on other planets (maybe even several of them). We've found bits of Martian rock on the Earth, which came from ancient impacts that caused bits of Martian meteorites to fling everywhere. In return, impacts have caused bits of the Earth to fling out into the solar system, which could find its way to Mars.
If Curiosity ever were to find remnants of life on Mars, the greatest irony is that it could have actually been from our own back yard, hundreds (perhaps even millions or billions) of years ago.