Actually weed is an immune system depressant which lowers your immune system and can get you worst. Also alcohol can dehydratate you and when you have a cold or the flu you need your immune system working to defeat the virus, plus you need to be well hydrated.
Pot has antibiotic properties, but antibiotics act only against bacterias and no viruses. The cold and the flu are viruses, only there is a cure for the flu(tamiflu) but for the cold there is no scientifically proven cure.