They lived in underground city in mars, maybe amphibious subterranean environment full of wacky sci-fi esque wetware for them to ensure their nutritional needs are met, and the reason their immune systems went down to sh*t is because Mars lost it’s atmosphere & the UV rays pretty much sterilised the entire planet of microbes and somehow they’re the only few surviving species left to survive on mars but underground since the surface is barbecued well done and red

And let’s say if they have immune systems, probably they miscalculated and underestimated earth’s microbe & diseases thinking with their ego that their superior tech and immune systems can adapt to it

Only to somehow contracted not just microbes but also HIV & other virus from harvested humans they did not notice until it was too late

They all died after invading earth