Singing with a Cold!
When I do masterclasses or career talks with young singers, I will sometimes go through a list of what I call “necessary skills” if you intend to pursue a career as a professional singer. One question I'm frequently asked is, “How can you sing if you don't feel well? How can you sing classical music if you have a cold!?”
And I tell an aspiring singer, basically: If you want to be a professional, you have to figure it out!
Our instruments are a part of our bodies, and they are affected by the stresses & strains & illnesses our bodies experience. But truly, if you wait until you're feeling perfectly healthy before you let yourself sing, you may never get to open your mouth!
I've been fortunate to have never had serious allergies or reflux that interferes with my singing, but in my opinion and in my experience, a singer whose voice is functionally healthy can still perform to a very high standard, even with a cold. Part of the (virtually lifelong) process of getting to know your instrument and what its capabilities are, is to learn how your voice responds when you aren't feeling absolutely healthy.
A few years ago, I had occasion to see a talented young soprano friend sing in her grad school production of Ravel's L'Enfant et les Sortilèges in the tricky and challenging role of Le Feu. It turned out she was actually running a fever and felt generally crappy, but she was able to rise to the challenge of that role's fiendish coloratura and turn in quite a good performance. Was it the best singing I've ever heard her do, or the best she herself had ever sung the aria? No, of course not. But she was able to make her instrument work, and work at the kind of high level that the average listener would never imagine that she was singing with “an indisposition,” as they used to call it.
So, you have a cold, but you still need to sing ... what to do? First, make sure that whatever your “indisposition” is, it's not something very contagious, or something that puts real stress on your vocal folds. In other words, an illness that doesn't really affect the SHAPE of your cords is generally something that can be sung through/over.
If it's mostly just nasal congestion you'll want to take some Dayquil, or a similar medication for cold symptoms. Stay hydrated, warm up carefully, and let your director and conductor know that you're feeling ill.
If you really feel like you can't do the staging required for the role, or feel too woozy to stand in front of the orchestra for the length of a concert, then you might want to call the company's general director/music director and let them know you may need to cancel, or to have someone on stand-by, if you're not sure. Obviously, for the benefit of the company and of your colleagues, you'll want to make that determination as soon as possible.
But if it's something you decide you CAN sing through you need to communicate with your conductor about it before the curtain goes up. Tell them, “I don't feel well tonight; I think we'll need to take the aria faster/slower.” Or, “I'm going to leave out some high notes,” or “I'll need to take some extra breaths” in such-and-such a passage. Things like that will alert your conductor to a need to pay extra attention as they accompany you at those crucial moments. Then go out and let your voice do the work you've trained it to do.
With a basically healthy vocal instrument, you should still be able to deliver 85-90% of your normal performance. Having said that, remember: We're not machines. If you're seriously ill and you really can't sing, then let the company know (again, as early as possible) that you’ll need to cancel for the night. They’ll appreciate being kept informed and also appreciate your consideration of your colleagues.