Earth organisms are not optimized for life off the Earth. A rational individual would expect major problems for organisms transported off Earth to zero gravity, or another planet.
Using this same logic, extra-terrestrial aliens would probably experience the same thing, calling into serious question the notion that UFOs are of extra-terrestrial origin. Of course, I am sure someone will explain it away — cryogenic travel, or leaping, or light speed travel not allowing time for health problems etc.
But is is one more logical data point suggesting there is no way we could be visited by aliens from other planets.