There is an entirely reasonable concern here, that any colonisation of Mars is going to lead to a population vulnerability. but the idea that humans on Mars would evolve away from having an immune system is silly to the point of insanity. That really just isn’t the way that the subject works out:
One change that could occur relatively fast? Non-Earth dwelling humans may quickly lose their immune system. In a sterile environment with no microorganisms present, the residents may have no need for a body capable of fighting germs. But this may not be such a bad thing, Solomon suggests it could be an opportunity to eradicate diseases, treating the ship flying to Mars as a sort of quarantine zone and ensuring the new inhabitants can lead healthier lives.
It’s this latter change that may force humans to eventually splinter irreversibly from their Earth-based counterparts. With no immune system, sex between Martian humans and Earthlings would be lethal. That could impose an artificial limit on how the two populations will be able to interact and co-mingle. The inability to form families or send offspring back and forth between the two planets could drive the two groups even further apart, assuming the whole issue of “who pays who taxes” hasn’t created an irreparable rift already.
Given how we absolutely have to have some vast amount of bacteria etc to make our guts work and all that the idea that we’re going to be in a sterile environment is nonsense. Given that we won’t be in a sterile environment then we’re not going to lose our immune systems.
There is an entirely reasonable point underlying this though. Which is that those immune systems will be primed by exposure to different things. We know this because it has happened before. We Europeans took smallpox, measles, even variations of ‘flu, to the Americas and got back syphilis. Many millions died on both sides.
We can also look at what happened to isolated populations as travel increased. The Faroe Islands are a test case for, I think, measles. Several times, over generations, the disease ripped through the population, killing double digit percentages. The timing of the next pandemic being around and about when a suitably large percentage of said population had grown up without routine exposure.
That sort of thing will indeed be a potential problem for any isolated colonies of humans up there and out in space.
But it’s unlikely to be sex which does it. It’ll be some variation of influenza which does. Down here we’ve 7 billion driving that toxic stew of evolution of that virus. It kills tens of thousands in the UK every year as it changes. A population which missed out on a couple of decades of that would be terribly vulnerable to someone landing with the sniffles.
It’s absurd to believe that humans will ever have a sterile environment, or lose our immune systems. But it’s entirely possible that some cut off community on some asteroid could all die of snot soon after being recontacted.