Here's how I see it: their immune systems were weak even when they lived on their homeworld and they experienced adverse effects when they colonized new worlds to which they adapted, right? Now stick someone like that in a sterile environment for a few generations and what was once weak gets weaker. To take their suits off requires a need to boost their immune systems to ensure that what could be deadly would be softened a bit to ensure that they don't die or become deathly ill: you build up your defenses to block a barrage of attacks. After doing the deed with Sheppard, Tali had a what sounded like cold. I'm not an immunologist by any stretch of the imagination, but I remember at least basic stuff to prevent sickness: take your vitamins, wash your hands, etc. Maybe not the exact same situation, but taking precautions to kill germs and boost your immune system to prevent disease.
And I remember reading something on here about doing gene therapy. It was mentioned briefly, but Tali said that going back to their homeworld rather than settling on a new world meant a difference of 60 years to 600 years respectively to re-adapt to living on a planet rather than in the germ-free Flotilla. Whether this was an exaggeration or not, it illustrates that they can have the technology to improve their immune systems but would be a wasted effort of sorts since their environment on the Flotilla is still sterile.
And I remember reading something on here about doing gene therapy. It was mentioned briefly, but Tali said that going back to their homeworld rather than settling on a new world meant a difference of 60 years to 600 years respectively to re-adapt to living on a planet rather than in the germ-free Flotilla. Whether this was an exaggeration or not, it illustrates that they can have the technology to improve their immune systems but would be a wasted effort of sorts since their environment on the Flotilla is still sterile.