Can microorganisms arrive “naturally” or accidentally from space? Can they survive in the hostile atmosphere of deep space? Can they ride in on meteorites or even space dust? How long can pathogens survive in a dormant state?

Pathogens can survive for many years and even decades depending on the nature of the organism, the substrate in which it resides, and the environmental conditions of storage or dormancy. There requires extreme luck for such organisms to persist for thousands of years. This is less likely on the part of organisms deposited on physical matter ending up in space and traversing the long distances necessary for it to reach Earth from another planet or even solar system. The odds are extremely remote that any known life form could survive such great spans of time in such harsh conditions. It is not just that the temperature is exceedingly low, approaching absolute zero, but the vacuum conditions of space, the inability of any organic material to retain moisture, is a further hindrance to viability so organisms could bounce back. This is remote in the extreme to begin with. Anything that comes from outside the Earth was brought to Earth as a part of a more benign environment within a transport vehicle of some kind that was capable of supporting life forms, so these would be organisms contaminating extraterrestrial beings themselves, or their material and equipment that might be jettisoned or exposed to Earth environment as tools, uniforms, and so on, carried or worn by extraterrestrial beings leaving their craft and carrying out various activities in the Earth environment and contaminating it accordingly. This has happened many times. Most such organisms are benign with regard to humans, but there have been introductions of pathogens this way also. There are similarities and susceptibility to many of the pathogens that affect human beings. This is because of similar biology among the extraterrestrials and human considered together. There are varying resistances, but some of the very deadly pathogens are able to attack all these species. These are the ones chosen by the extraterrestrials to be introduced deliberately as a scourge for humanity. The inadvertent introduction of such organisms is relatively rare because the extraterrestrials do control their local environment with regard to any kinds of hazards because they have learned how to do this long ago. So any pathogens brought by them will be a deliberate act to inoculate a dangerous species with not only reckless disregard for the consequences, there is also a depraved desire for this to happen.