I was wondering the same thing when I was recently sick with a cold, and I understood that weed can weaken the immune system temporarily. I stopped puffing for a couple of days but I'm a regular toker, so it was hard to sleep without having puffed during the day. I didn't feel much better and sleep is way important in getting over a cold, so I blazed up again(vaped), and it actually cleared out my lungs a bit. This was in the final stages of my cold, so this isn't typical. I think vaporizing sparingly only when needed for sleep is okay when you're sick and it won't prolong the sickness too badly.