The new era of 'sample return' missions in space exploration has two downsides: "back contamination" - the risk that samples returned from other worlds such as Mars - just might - contain alien germs capable of turning into a worldwide plague, or at least wreaking havoc with the Earth's natural environment, and "forward contamination" - the possibility that spacecraft might contaminate the worlds they land on with Earth microbes, destroying scientifically priceless alien lifeforms before we even have a chance to study them.
NASA has planned a new era in space exploration with an agenda of collecting samples of material from worlds more distant than the Moon. However, this plan runs the risk of both forward contamination (the possibility that spacecraft might contaminate the worlds they land on with Earth microbes) and back contamination (the risk that the samples might contain alien germs capable of turning into a worldwide plague, or at least wreaking havoc with the Earth's natural environment).