I'm an asthma sufferer and weed has definitely not harmed me from smoking it. I always breathe better afterwards. Weed dilates the bronchioli (larger vessels) of the lungs as well as the aveoli sacs, as mentioned above. It also has been shown to have antibiotic properties, meaning a healthier immune system. If you're suffering a repiratory infection, restrain from smoking it and make some brwonies or something (someone may have already mentioned this.. I'm really stoned and don't remember lol). As for general allergies you just have to know your body and how much you can handle. Hope this helps!