Ostensibly, via the "Hey Barkeep!" feature on this site we answer questions about beer that others are afraid to ask. Today, though, is a bit of reverse "Hey Barkeep!" and I'm the one asking the questions. I know, with relatively high anecdotal certainty, that drinking when you have a cold is a bad idea. But, I will be the first to admit I know nothing about medicine and I am not afraid to ask.
So, the question of the day: why shouldn't I drink beer (alcohol) when I'm sick?
The hardest part about answering medical questions online is the inability to trust anything you read here. These internets are chock-full of quacks and frauds and hucksters (we'll ignore the catcalls of naval-gazing from the gallery, thank you very much). So, I will preface everything I say here by saying, if you are a doctor and what you read here is incorrect, please get a hold of us and correct us, or post a comment correcting us.
There are two primary reasons you should not drink when you are ill: 1) alcohol dehydrates you; 2) alcohol may suppress the immune system.
The American Lung Association tells me that when I have a cold, I should stay hydrated. Proper hydration ensures that mucus remains moist and easy to clear and keep the nose and throat from drying out. Beer is liquid, right? Well, not really; beer (and caffeine, apparently) dehydrates. Basically, alcohol prevents the brain from signalling the kidney to retain water. So, the kidneys actually release rather than retain liquid, which only compounds the problem.
Moreover, alcohol, particularly in quantity, inhibits the immune system. While its effects in low levels does not appear to have much effect, an amount sufficient to cause intoxication could exacerbate problems. Alcohol in such quantities can inhibit the ability of white blood cells to multiply and decreases the efficacy of white blood cells.
Not to mention, the symptoms of the common cold (congestion, weakened senses of smell and taste, runny nose, scratchy throat, etc.) inhibit the appreciation of beer. If you can't smell and taste, that $23 bottle Nøgne Ø Dark Horizon will be wasted (by the way, for those interested, this is an awesome Russian Imperial Stout and can be purchased at Steve's Liquor on University). So, please, if you are sick, put down the beer for a few days, go get some non-caffienated tea (chamomile is my weapon of choice) and take some real cold medicine.