Alien Origin (speculations)

[The Necronoir]

I don't understand why Space Jockey associated with Aliens as creator in some theories. And Aliens as bioweapon are doubtful, because deadly virus as bioweapon is more effective than difficult parasitic lifeform...

Yes an no. Certainly a virus is more easily spread, and could very well be more effective, but it's also much harder to control once it has been released. At least with the aliens as a weapon they are able to be tracked visibly and readily moved around. It's also difficult to believe that a virus could be devised which would effect the great variety of species we've seen the alien encounter thus far. Even on earth there are very few virus which effect any great range of species.

Finally, if the aliens are a bioweapon designed by the pilot race then there's no reason to believe it was their only creation of that sort. They could use a whole range of biological agents as weaponry alongside the aliens, including both virus and bacteria.

[Valerios]

its no need to kill every form of life in a planet, for example on earth its enough to eliminate humans not every mammal, even though aliens are easy to seek and destroy in some cases eggs left in a tropical jungle/islands  or in deep canyons or in mines/caves/building basements that aliens used as their refuge during their attack in the new world are far more difficult to strerilize than a "normal" bio-weapon

[The Necronoir]

its no need to kill every form of life in a planet, for example on earth its enough to eliminate humans not every mammal, even though aliens are easy to seek and destroy in some cases eggs left in a tropical jungle/islands  or in deep canyons or in mines/caves/building basements that aliens used as their refuge during their attack in the new world are far more difficult to strerilize than a "normal" bio-weapon

Again, not really. Even in the very worst scenario an alien infestations could simply be eliminated with a combination of explosive bombardment and terrestrial mopping up. If a virus is loose in the atmosphere how exactly are you going to control that? They have no life-span, they're microscopic and thus difficult to track, and there's no way of knowing whether you've ever completely eradicated it or not (just look at small-pox). I'm not sure even completely irradiating a planet and its atmosphere would even guarantee destroying a virus, seeing as they are only partial DNA sequences, and exist in stasis outside a host anyway. Even if that did work you'd make the place completely uninhabitable for yourself let alone the original inhabitants.

[Valerios]

And again if you create a virus agains lets say Dogs that doesnt mean that the virus kills humans also... in not trying to do a lesson here but im speaking in a more conservative way... do you really think that aliens spread in a jungle are easily eradicated by bombs/napalms etc? do i have to remind the Vietnam case? the Afganistan rocky mountains? make a research about the BLU-82 bomb (27.000$) cost each, used to clear jungle in Vietnam for helicopter landing grounds, Guerrilla attacks? The Predator 1 movie is the perfect example were 1 predator eradicated an elite team easily, think about 4-5 aliens what they would do (and by the way you do not have to burn-out the atmosphere to kill a virus, a vaccination (the administration of antigenic/antivirus material (the Vaccine) to produce immunity to a disease.) is enough