interaction is usually irrelevant to the existence of the producing organism but may be damaging or even fatal to the infected organism.

Infectious agents, which may be actively or opportunistically invasive, must multiply in or on the host in order to cause damage. The capacity of a microbe to infect a host requires an intimate interaction between the pathogen and the host and often depends on highly specific interactions between cell surfaces of the host and pathogen. Above all, infectious agents must overcome the defenses that have evolved in most potential hosts as a consequence of persistent, unremitting challenges by potential pathogens on Earth. Living organisms defend themselves mechanically and chemically in diverse ways against agents that are themselves constantly changing.

The chances that invasive properties would have evolved in putative martian microbes in the absence of evolutionary selection pressure for such properties is vanishingly small. Subcellular disease agents, such as viruses and prions, are biologically part of their host organisms, and an extraterrestrial source of such agents is extremely unlikely.

Although hypothetical extraterrestrial biota could have properties different from those of Earth biota, it must be assumed that the chemical reactions governing their metabolism would be largely the same. Putative martian microorganisms would likely be functionally similar to some of Earth's soil bacteria, but because the range of habitats available on Mars is much narrower than that on