IBS is exactly what I did mean, though other ailments would fit too
We have similar immunity trouble here, DD struggles with IBS & gets every bug going, often more than once :( my immunity isn't great either, though I did get my IBS under control years ago, but have other health problems that affect immunity, so I'm probably not the best example.
I would highly recommend taking the pre/probiotics, even if your IBS is controlled, your gut flora will be out of balance & they will help sort that out - if IBS not controlled, you will need to sort that out first -
With IBS chances are even if you eat well, your gut doesn't function properly so you aren't getting the nutrition from your food & the flora that helps that will be continuously messed up - omega oils ( which you get naturally in oily fish or flax seeds etc) will help with general health & immunity further & Spirulina is packed full of vitamins, minerals etc & is very easy to digest, it has immune boosting enzymes too - its worked wonders for DD :)
Maybe post a separate IBS thread, as what helps one, doesn't always work for others, but diet changes, turmeric, blackstrap molasses have helped DD & I found the herb cats claw to be very good for myself along with diet changes etc too
HTH