I see no reason that something alien would even react to terrestrial life. Why would it be infectious?
Read up on the theory of panspermia http://www.panspermia.org. Primary researcher is a Dr. Wickramasinghe.
In a nutshell, the idea is that life is not native to Earth. Instead, some of what we see as vast stellar clouds of dust are huge clouds of bacteria in a hibernating state. When they come into contact with environments that come into being throughout the ages, they come back to life and colonize whatever habitat they land in.
Under this idea, we might find bacteria in places all over the solar system that can provide the minerals and materials necessary for bacteria to grow.
In the last year or two, this theory has gained some credibility. Recent high-altitude labratory ballons/planes collected samples of bacteria high in the tropopause, far above any of the atmospheric circulations that can conceivably bring bacteria up to those levels. While of course such evidence is not yet conclusive, it is very thought provoking, similar to the possible bacterial remnants found in that Mars meteorite in 1996 from Antartica.
How this relates to the topic is that the bacteria collected in these experiments closely resembles that of terrestrial bacteria. This means one of 2 things.
1)the bacteria *are* airborne, Earth evolved life, so nothing to worry about.
2)they actually did come from space, meaning that life throughout much of the nearby galaxy would likely be based on the same biochemical origins. If so, then its very possible that diseases from bacteria on Mars could cause illness in terrestrial forms of life. Also means that such dieases might also respond to anti-bacterial type medicines we've developed over the years. A bit of 2-edged sword.
Panspermia is an interesting idea, and it has *big* implications if its ever proven to be true.
Geuis, I have look at web site and their some errors and the guy should uptade himself on evolutionary theories. For example, he still refer bacteria and archea as eubacteria and archeabacteria respectivly. These last term are rarely used anymore because it assume that archea are ancestrial to bacteria and that everything evolve from bacteria.(Check this link, the article explain what I mean http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=2112744). The guy loses a lot of credibility.
The theory that pathogen could invade earth is a bit week. If you think about it, pathogenic virus and bacteria tend to be host specific (i.e. bacterial virus do not infect human, and it can go down to species specificity). Therefore evolution with a potential host tend to bring pathogenecity and I doubt that extraterrestrial live form have evolve with humanoid (unless the bacteria left the earth couple of thousand years ago and it is coming back ). The probably that I see is that the E.T. virus or bacteria could be bacterial pathogen. This could cause decimate our intestinal flora and make us more prone to disease.
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