Back Contamination is the idea that bacteria on Mars (if they exist) may be highly dangerous to Earth life. Therefore, extraordinary precautions must be taken to prevent Mars life from reaching Earth. This is a ridiculous idea for several reasons.
- 1 Earth and Mars have contaminated each other for eons
- 2 Disease organisms must evolve with their hosts
- 3 The Martian surface is unlike Earth
- 4 Real costs associated with fear of Back Contamination
- 5 Other regions of dangerous contamination
- 6 Life on Earth is already doomed
- 7 Logical arguments for supporting taking extraordinary steps to avoid Back Contamination
- 8 References:
Earth and Mars have contaminated each other for eons
The meteorite Allen Hills 84001 was knocked off Mars by a large meteor impact. It travelled thru space, eventually landing on Antarctica. Meteorites on an ice sheet are easy to find (no local rocks) and AH84001 was the first meteorite found in that collection season. There were microscopic traces which looked very much like life. Studies of the rock showed that inside cracks, the local temperature never rose above 50C during its ejection from Mars and the entry thru Earth's atmosphere. This meteorite proved that bacteria, and bacteria spores, could travel from Mars to Earth safely. 
Very large impacts on Earth likewise can send unsterilized rocks to Mars. The dinosaurs killer asteroid which hit 68 MYA would have sent unsterilized mud and gravel into space, some of which would eventually land on Mars.
Panspermia is a study that says events like these can spread life between worlds in a solar system, and potentially to other solar systems. Note that this happens relatively rarely now, but was more common earlier in the solar system as large impacts were more common.
Bacteria certainly have moved from Earth to Mars, and if life ever was on Mars, it would have travelled to Earth. If Mars has bacterial life, it has infected Earth many times in the history of our planet.
Disease organisms must evolve with their hosts
I am more likely to contract a bad case of Dutch Elm Disease than get sick from Martian bacteria. At least Dutch Elm Disease has evolved in Earth like conditions.
I (Richard Smith) think that it is likely that bacteria live on Mars, deep underground, in cracks in the rocks. They survive from higher energy elements created from geologic processes and radioactive decay. Such bacteria have been found deep within the Earth making up about 15% of the total biosphere mass on Earth.  Such deep bacteria are of no threat to Earth surface life. This is demonstrated by the failure of plagues breaking out every time an oil well is drilled. The bacteria deep underground have evolved to enjoy the conditions deep below, and would be killed by the high oxygen environment up top. Nor do such bacteria 'know' any of the tricks needed to avoid our immune system.
The Red Queen Hypothesis,  suggests that hosts and pathogens are in a never ending race evolving defences and counter measures to prevent or allow parasitism. (Both sides must run as fast as they can to stay in the same place.) Basically, disease organisms are not random bacteria, but rather are 'programmed' precisely to overcome the defences of the hosts that they victimize. Therefore, Mars won't have any disease organisms which target mammals (or Dutch Elm Trees), because there are no mammals (or Dutch Elms) on Mars for them to evolve inside.
The Martian surface is unlike Earth
If bacteria, could somehow live on or near the Martian surface, they would have evolved to survive in a near vacuum, high radiation, extremely arid, extremely cold, high salt, low gravity, high perchlorate environment. If Earth bacteria were to be introduced onto the Martian surface would we expect them to outcompete Martian bacteria on their home ground? Likewise, Martian bacteria would have massive amounts of genetic machinery which would NOT help them survive on the high pressure, low radiation, wet, hot, low salt, high gravity, perchlorate free environment of Earth. Much of their genetic code would be of no use, or a hindrance to them here.
The idea that they would run rampant is impossible.
Real costs associated with fear of Back Contamination
Some people have suggested that the Mars Sample Return mission should NOT return its samples directly to Earth, but rather they should be sent to a special laboratory in Earth (or Lunar!) orbit to check that they are safe. A wide variety of prices have been suggested for such plans, some costing $500 million dollars. Consider what $500 M could do if spent on health care here on Earth. It would save or materially improve the quality and duration of life for hundred of thousands of people (especially if spent in the poorer nations with marginal health care).
There is a real life cost to supporting this silly idea. It is morally and ethically wrong to waste millions of dollars on it.
Other regions of dangerous contamination
It would be more logical to outlaw gardening than pass regulations preventing Back Contamination from Mars. We know for a fact that the soil and sub-soils of some parts of Earth contain disease creating organisms such as e-coli, staph, rabies, anthrax spores, tetanus, and others. Such soils may contain spores of death cap, webcam, autumn skullcap, destroying angels, conocybe filers, or deadly dapperling mushrooms, (the most deadly mushrooms on the planet). Gardening can expose you to insect bites such as ticks which can carry Lyme disease, or malaria carrying mosquitoes. (Gardening can expose you to dangerous radiation causing sunburn, tho this is not a biologically infectious risk.) Could not some horrible disease from the age of the dinosaurs be lurking underground in our back yard? Logically, gardening would be a better place for governments to regulate for the health of our planet.
Scuba divers often retrieve souvenirs from their undersea explorations. Shells, interesting rocks, bacteria infested wreckage, or bits of coral! ALL unsterilized! Horrific! How do we know that some disease from the deep might not kill off all humans? Bacteria which evolve underwater are far more likely to find the inside of our bodies a welcoming environment than something optimized to live on Mars.
Spelunkers go underground, in damp dark environments teeming with bacteria. What are the chances that some bacteria which has evolved in damp, dark places (like the inside of a human body), will kill us all? Caves are far more likely to have disease organisms than the surface of Mars.
Life on Earth is already doomed
Earth is on the inner edge of the habitable zone around the sun. In other words, if the sun was just a bit hotter, we would have a runaway greenhouse effect, which would sterilize the surface. However, the sun is slowly and steadily getting hotter as it burns hydrogen (via fusion) and builds up the denser helium 'ash' in the core of the sun. This causes it to shrink, and burn hotter as the core of the sun, (where fusion occurs), becomes more dense. Within 500 million years the Earth will be so hot that the oceans will boil away, and multicellular life on Earth will die out. (The bacteria deep underground will likely survive until the Earth is destroyed when the sun becomes a red giant.)
If the complex lifeforms of the Earth are to survive, they must spread to other planets. The universe is filled with sterile rocks. The purpose of life is to live, and spread to previously sterile environments.
Logical arguments for supporting taking extraordinary steps to avoid Back Contamination
Some have written that tho we can't expect pathogens to exist on Mars, perhaps something is there which will prove to be really bad. Maybe something evolved to live on Mars will outcompete Earth organisms on Earth? Maybe something from Mars will cause problems on Earth? Can we be sure?
If that is the concern, then we should outlaw meteorites, because bits of Mars are falling on Earth every year. It has been estimated that approximately 100 kg of Martian rocks, gravel, and dust, fall on Earth every year. (This estimate is based on ~0.00385% of meteorites have been identified as coming from Mars, and on the total mass of extra terrestrial material falling to Earth.)