For much of history comets have been associated with death and disease. There is increasing evidence that that viruses, microbes and other living creatures dwell beneath the surface of comets meteors, asteroids and other stellar debris and when these extraterrestrial objects pass close to Earth, or strike the atmosphere of this planet. If the meteor or comet disintegrates then trillions of microbes and viruses may survive and then slowly drift down from the upper atmosphere. For the last 50 years different teams of investigators have discovered microbes, bacteria, fungi and micro-biofossils in space dust collected from the stratosphere at heights of up to 61km. These microbes are essentially the same as those found on Earth. If they are extraterrestrial in origin, then possibly the genetic code is not just universal on Earth, but throughout the cosmos; meaning that DNA is a cosmic imperative for life. However, it is also know that microbes live in the atmosphere, and it is possible that these microbes were somehow blown upward into the stratosphere; perhaps by cyclones, hurricanes, volcanic eruption, or powerful solar winds. In fact, on Earth, or other habitable planets, powerful solar winds could eject microbes and viruses into space. Therefore, other planets could be repeatedly infected by the survivors. In our own solar system, this could include Mars. Liquid water has almost certainly been a feature on Mars in its earlier history, and the presence of extinct or present life on Mars cannot be excluded. If there is life on Mars, and even if it originated in space, or from Earth, it can be deduced that would be decidedly different from their Earthly counterparts and therefor posing a risk of disease or contagion to future astronauts who visit the Red Planet. Based on our current understanding of host-pathogen relationships and evolutionary processes, we may conclude that the chance of a human mission to Mars to encounter pathogenic microorganisms is a real possibility.

Bacteria, archae, and viruses represent the most numerous, diverse, and stress resistant life forms on this planet, and are the perfect candidates for withstanding and surviving the physical stresses related to interplanetary travel include the ejection into space, exposure to the space medium, temperature extremes, lack of water, and radiation of space, as well as a crash landing onto the surface of another planet. Once in a new location, surviving organisms may be forced to compete with the inhabitants of these planets, or they may infect them. If passing comets have continued to deposit viruses and microorganisms on this planet, this may explain why ancient astronomers and civilizations attributed the periodic outbreak of plague to these stellar objects. Moreover, the subsequent evolution and extinction of life may have been directly impacted by the continued arrival of bacteria, archae, viruses, and their genes from space. On this picture the evolution of higher plants and animals, including humans, would be impacted by the insertion of genes from space, as well as recurrent episodes of pandemic disease. Near-culling pandemics and extinction episodes have in fact been preceded by or followed by inserts of viral genes into survivors who have transmitted these viral elements to their progeny, thereby impacting future evolution. Although ancient fears and reverence of comets may be coincidental with the outbreaks of pandemics, they may also have a factual basis.

Contents
1. Comets and Contagion: Evolution and Diseases From Space
2. Tunguska: Comets, Contagion and... 3. Are Microbes Currently Arriving to Earth from Space?
4. Identification of Micro-Biofossils in Space Dust
5. Microbial Survival Mechanisms and Interplanetary Transfer of Life Through Space.
6. Infection Risk of a Human Mission to Mars
7. Meteors, Microbes, Viruses: Genetic Seeds of Life Keep Falling to Earth
8. Viruses, Evolution, Interplanetary Horizontal Gene Transfer

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