I've heard probiotics can be harmful to those with weakened immune systems. i'm getting over bronchitis - can i take them? not on antibiotics.

Should be okay: Probiotics are organisms, not "good or bad", that supplement your colonic flora. Peer review literature shows 1 in 4 patients treated are clinically improved with probiotics. Different probiotics benefit different symptoms. Probiotics are generally harmless, but they are not a panacea ; their use should not substitute for a good medical evaluation followed by appropriate directed therapy.