In Russia, Anton Shkaplerov, who will lead Russia’s International Space Station crew in December, said that samples from a recent swabbing of the outside of the ISS shows living bacteria and microbes that weren’t on the module when it initially launched into orbit. Many are speculating if that means alien microbes have reached the ISS, but others say a more likely explanation is contamination from humans or earth-based organisms that can survive in outer space. National Geographic said that a TASS report showed that “on a previous ISS mission, bacteria accidentally hitched a ride to the station on tablet PCs and other materials,” and survived on the outside of the station for three years, even though temperatures fluctuated between -150 and 150 degrees Celsius.
Total sterility is impossible and even with stringent guidelines from NASA to limit microbial contamination on flight systems, humans easily spread their germs and microbes even unintentionally.