The jury is still out on whether or not marijuana impairs the immune system.
According to the
National Institutes of Health, research shows THC has a negative effect on various immune cells, but there are no human studies to date which connect that effect with increased infection.
A
2002 study found that marijuana certainly has an effect on the immune system, "however, because of the complexity of the immune system, we can't say yet whether the effect we've observed in humans is good or bad," UCLA scientist Thomas Klein wrote.
Animal research seems to suggest trouble, but the NIH says the matter warrents more research.