pineapple YEA!

Impossible.

Due to common sense, the planet may already have pre-exsisting life on it's surface.

Where there's life, theres diseases.
Where there's diseases, theres getting sick
Where there's people who have no immunity to the disease, theres eventual dying.

You cannot expect to put someone on a foreign planet and expect them not to contract a disease they are not immune to. Thats how the british killed hundreds of thousands of other people in the 1600's to 1800's with smallpox.

If there are diseases on this planet, let alone life that feeds off of other life. The chances are any bacteria there will feast upon us like fatpeople eatting big macs at a mcdonalds.

You've already made the enormous leap that these alien lifeforms have a biology compatible with earth life. That's a ridiculous assumption to make. Even here on earth, most diseases only affect a small portion of the wildlife. More than likely the result of alien bacteria meeting earth life would be "I don't know what to do with this."

That's not to say we shouldn't be careful exploring it (After all they could have toxic byproducts of their biology), but trying to equate humans from one continent meeting humans from another continent to life that developed independently on separate planets with potentially radically different biologies is absurd.