Let's hope we don't encounter life on Mars, one pandemic per lifetime will suffice, thank you.

You're conflating life as hosts and parasites. Most parasites require co-evolution with hosts. In the case of pandemic viruses, none existing anywhere else in the universe have the slightest chance of infecting us because there is absolutely nothing evolved exactly like us anywhere else.

Or a Martian microbe might just eat through any Earthly thing unable to fight back. Gray goo

Mars... no, obviously that would have to be "red goo"

So it's incorporating large amounts of iron into its system? Take out the magnets and mess that thing up.

Astronauts are routinely quarantined:

https://en.wikipedia.org/wiki/Mobile_quarantine_facility

You realize that the apollo missions were done like 50 years ago, right?

You are absolutely correct that we have not returned to the Moon in 50 years.

They were quarantined in case they bring back some unknown pathogens we don't know what do with.

There are plenty of things that didn’t evolve to harm us but which can nonetheless harm us. Meteors are an obvious macroscopic example, while toxic chemicals are an obvious microscopic example.

I understand that the boundaries are blurry but this particular comment was talking about life harming other life.

Right, and my point is simply that if inorganic chemistry is more than capable of harming life, it’s a bit strange to assume that organic chemistry and life would somehow have less capability to do so.

I think it's pretty likely there is bacterial life under the surface of Mars.

1 km down should be both warm and moist enough.

Do you really mean today bacterial, and not microscopic or something like it? Bacterial implies a common ancestry with bacteria on earth. That, IMO, is less likely than finding very small life forms that are dissimilar from anything on earth.

Yeah, I like the idea that bacterial life spread between the planets.

We have found Mars fragments on Earth. Some almost almost certainly went the other direction.

You like the idea, that's fine, but if I were to make a bet on PredictionBook... .0001%.

Fragments don't just lift out of a fully formed planet without a collision with another massive body. I assume the Martian fragments you mention are from the early solar system when things were still being formed. Also if something like the Chicxulub impact was enough to lift fragments out of Earth, I assume such fragments would have gotten so hot that any microbes on it just died, to say nothing of the trip in the void of space to Mars which would likely take millions of years before finally crossing paths with it. Good luck surviving the solar radiation and lack of atmosphere in the interim...

I agree that it's a very unlikely individual event.

But it only needs to happen once, with one bacteria surviving the trip over the billions of years when both planets were hospitable to life.

I'd be hesitant to bet against those odds. I'm also not betting for it. Just being open to the possibility.