With so many probes heading for Mars and other planets, the question of whether they could bring back new diseases has become important. SARS, Mad Cow Disease and HIV are only three of the diseases that have crossed the species barrier, so infectious pathogens from Martian rock samples probably could too.

Leslie Mullen writes in Astrobiology Magazine that the International Committee Against Martian Sample Return is worried about this. Not all pathogens cross the species barrier?our dogs and cats get diseases that don’t affect us. Chicken and sheep farmers are untouched by diseases that wipe out their flocks and herds. A Martian microbe could enter the human body, but be harmless because it’s incompatible with human physiology.

“After living in the dirt of Mars, a pathogen could see our bodies as a comparable host; they could treat us ‘like dirt,'” says NASA?s John Rummel. “?It could be that even if the microbes lived inside us, they wouldn’t do anything, it would just be this lump living inside you.” This is the most likely scenario, since life on Mars would have evolved without humans present. Pathogens are most likely to cross species barriers when both species evolved at the same time.

One theory is that life was brought to Earth by meteorites coming from Mars, suggesting that life on Mars might be closely related to life on Earth. However, life forms on both planets would have evolved extensively since then, meaning that pathogens from one planet might no longer be able to infect living things on the other planet. However, if we are related to Mars life, it could also mean we’re susceptible to Mars diseases.

Can we wall up evil and keep it from getting out? The Egyptians tried, and failed. Learn all about it on this week’s Dreamland!

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