I saw the common cold statted up in the Dragonstar Player's Companion from Fantasy Flight Games. How would the common cold affect player characters in D&D 5e?
I was thinking it would be -1 to str and dex to represent basic aches and pains.
I think that the common cold should be kind of trivial to adventurers whose constitutions are likely several standard deviations above the mean. In the context of D&D diseases, getting a cold seems more like an annoyance than anything else.
If you want, though, you could use the exhaustion mechanic (PHB291), which is a pre-existing mechanic that's well-suited to this. A single level of exhaustion gives disadvantage on ability checks, which might be consistent with a severe cold.
Given the overall "heroic" tone of modern D&D (doesn't even account for reasonable maiming due to battle wounds, infection from same, etc.), I'd be strongly inclined to leave out piddly stuff like colds, indigestion (unless specific to Plot), corns, bunions, and ingrown toenails, and so forth. Heroes should have heroic afflictions, diseases that will kill you if a quest isn't completed, double-time -- or no ailments at all, short of that.
Take a page from an existing Published Adventure: "Out of the Abyss"
Out of the Abyss (OotA) has this disease called Grackle-lung. It involves:
It should be noted, however, that the DC for resisting Grackle-lung is pretty low (DC 11 for the first time, and DC10 + exhaustion level after that). And that the disease is a side-effect of breathing in the smoggy smoke of an underground Duergar city with forges constantly alight with Red Dragon fire 24/7, and that the description of the disease says that the phlegm coughed up by victims are black as soot- so a common cold would be considerably weaker.
Is the common cold really weak enough to not make a difference? If the DC for a disease like Grackle-lung is already pretty low, would that mean the Common Cold is less than 10? Maybe 5? Do you really want to roll a constitution check every time a PC casts a Verbal spell or takes the dash action just to beat the DC of 5? This is for you to decide, I'm just here to tell you there's a disease that does that, but it's pretty exceptional.
We tested the following as an experiment, but not extensively because we decided it wasn't worth it. Still, it worked, so I offer it for people who would think that this could be fun enough to work into the regular flow.
Imagine how it might affect verbal components of spellcasting. I imagine that losing precision when invoking the magic words would possibly cause the spell to fail, or be cast as something else (unintended consequences). Try Disadvantage.
All that coughing and sniffling can't possibly be good for stealth. You can somewhat control it, but when you get down to attempting something you are more likely to fail — Disadvantage.
If a Perception check involves the sense of smell (e.g., the odor of a goblin squad wafting down a corridor), then you can also try Disadvantage.
You could also reduce the number of hit points and spells recovered when resting, sort of like how you don't feel up to par when you wake up with a cold.