I don't know if marijuana weakens the immune system, but smoking definetely does (or I get sick a lot more). You can argue all you want, but the fact is, smoking is unhealthy. If you have the money (and time/place), definetely cook some marijuana brownies/muffins/cookies. Marijuana is terrific when you're ill (at least for me), but smoking pretty much negates all the positive effects you get from it.
If possible, eat it (it will help). If not, don't smoke and go without (and get some other meds).