Adding a couple of coppers to Tom's pile....

It seems to me that there are myriad other sources of possible pandemic-causing infectious agents... some botanist digs up a tree root sample in the Andes and releases a previously unknown bacteria; an archaeologist unearths a tomb in Mesopotamia and is exposed to a virulent virus from 3500 BC; some 12 year old kid working in her basement lab concocts a brew that mutates Herpes simplex virus into something that makes anthrax, Ebola and Spanish Flu look like the sniffles; an unmanned probe brings a bit of dirt back from Mars and it contains a deadly organism; Coca-Cola is found to contain an infectious form of sucrose that incubates for 50 years before causing an agonizing death by overstimulating the laugh center of the brain.

My point is that the odds against pretty much anything happening are not zero, but we can't refrain from doing things that have any kind of risk associated with them or we wouldn't do anything at all. Focussing on remote possibilities of dangers from Mars organisms is a sure way to promote hysteria among the general public and purse-string tugging politicians.

A last point: my understanding of disease is that most if not all infectious agents, be they parasites, fungi, bacteria, viruses or prions, have co-evolved with their hosts and that one cannot exist without the other. Since Mars and Earth organisms have been evolving separately for millions of years, if not forever (which I think is the case), there is no relationship between organisms (if the Martian ones even exist) that could allow infection.

Is anyone aware of any infectious agent that crosses more than Class boundaries on Earth? (other than things like malaria with differrent species as vectors, but in this case the organism is acting like two different diseases depending on the host, and has evloved with both hosts over a long time period).

A far greater danger than infection, IMHO, is that Humankind shrivels up under the threat of fear-mongers and never ventures out of the womb.