It is highly likely that there will one day be a species that visits our planet whose home level of radiation is so strong, that a few moments in their presence will give those that meet them a lethal dose of radiation. As the aliens themselves are radioactive. Unfortunately it will kill the poor humans exposed. The humans that witness the alien's "oops" moment, or as they will call it "massacre" will assume the worst, playing off emotions, adrenaline, and fear screaming out, "they attacked us!" Likely a very short war will start between the two species. The aliens will never actually fire a real weapon at humanity, and just simply drop a few bricks made from the soil from their home planet on the attackers which will be so radioactive they will be as feared as nuclear weapons themselves. Only until the communication between our two species and peace is sought they would just continue letting the most basic levels of their worlds radiation wipe out the offenders. The aftermath of the war will be a species that has a future home on the planet in those historic war zones, as humans cannot live there anymore, thus creating an alien world's embassy zone on our planet. It's likely that only until afterwards humanity will open their eyes to the possibility that this endless universe is pretty much exactly that.. That all possible concepts have to be considered.. Even the concept that meeting an alien could be so damaging from it's radiation alone, and that we should not be fools to shake the hands of such beings.. as the microbes themselves could be as deadly as arsenic. But alas, the fools of humanity will find themselves in such positions while the creative intelligent minds of humanity would already have thought of such things. Which is why we would keep our distance when they landed. Then after the ignorant are erased from history, the intelligent will step forward. In all reality that sort of specie specific evolutionary cleansing is all for the greater good of humanity anyway.