You might be familiar with the feeling: you've been singing well, holding up through all of your rehearsals and lessons, and your voice is doing swimmingly. Then, one morning, you wake up with a sore throat. Or a drippy nose. Or you suddenly feel like you've run into a brick wall and barely have the energy to get out of bed.
In short, you fear you're getting sick. And for every singer everywhere, this (sometimes justified) paranoia puts us into CRISIS MODE.
Every singer knows that even "just a cold" can make your vocal life a living nightmare. For a singer, there is no "just a cold" when your throat is sore, your face is leaking, and you feel like you're hacking up a lung.
In a perfect world, we wouldn't have to sing while sick at all. Even in our imperfect world, you should avoid singing while sick if you possibly can. Alas, however, sometimes circumstances arise in which singers are faced with the decision of whether or not to sing with a sick voice. It can be hard to know what decision to make. This is partly because everyone's voice and body is different, and partly because sickness can be so unpredictable. Your decision about whether to sing should weigh several things: the nature of your sickness, the nature of the rep you're supposed to sing in your next lesson/rehearsal/concert, and your personal knowledge of your own body and how it typically deals with sickness.
Before I go any further, please understand that what I am about to give you in this post are GUIDELINES ONLY. What I am about to say here is based on my knowledge of how the vocal mechanism works and on my personal experience, but I am a singer and voice teacher, not an ENT or laryngologist. I cannot see your vocal folds and thus cannot and do not prescribe any specific course of action for your particular situation. Singing while sick can be a very risky business, and it's imperative that you make the decision to do it under the guidance of someone who knows you and your voice well. If you are unsure about what your voice needs, ALWAYS consult your teacher or coach, or an ENT or other voice specialist.
Now that that's cleared up....
Virtually all voice professionals would agree (I hope) that you definitely should NOT sing if:
1. You have any malady that directly affects your thoat or lower respiratory system (strep throat, laryngitis, swollen glands, bronchitis, mono, or a hacking cough).
2. You have the stomach flu. Vomit really wreaks havoc on your folds.
3. You have anything else that severely inhibits the amount or quality of sound you normally put out.
If your sickness is basically a head cold (sinus congestion, runny nose, etc.), the situation can be a bit murkier. It MAY be okay to sing with a head cold IF there is nothing wrong with your throat, your lungs, or your tummy. I'm not saying that singing with a head cold is a walk in the park, or that you won't feel as tired as if you ran a marathon after, or that your voice won't fatigue more quickly than usual, but my point is that it could still be SAFE to sing - even if it's not COMFORTABLE.
Consider the type of repertoire you're scheduled to sing in the next few days. Unless the sickness has temporarily rendered you completely mute (in which case you should not be singing at all), there will likely be an area of your voice that will seem to function more or less normally, and an area (or areas) that seems to have fallen victim to your body's virus. If your upcoming concert program is full of musical theater and your speaking voice (i.e., your TA-dominant register) feels weak or crackly, then bow out if you can. If you're singing something classical in this situation, it's possible your head voice may still function properly enough to sing the rep.
Also consider what "stage" you are at with your sickness. If your colds are like mine and always progress in a certain way (i.e., symptom A eventually gives way to symptom B, which eventually gives way to symptom C, etc.), you can make a somewhat informed calculation of how you might be feeling in one or two days' time, allowing you to plan your singing (or not-singing) a little bit better.
If you do choose to sing, there a few important things to keep in mind:
1. Your sick voice is not going to sound or feel like your healthy voice. Don't beat yourself up for sounding "less than your best." Instead, set a small, attainable goal for your performance, such as "I'm going to make sure to support that high note properly" or "I'm going to make sure my [vowel x] is aligned properly."
2. Don't overdo it. Your voice, when it is sick, will not have the stamina that your healthy voice has, period. If you sing too much or too heavily when you are sick, you will no doubt pay for it for the next couple days. So be very, very careful.
3. The stronger your technique, the better your voice will hold up under sickness. This is why we practice for hours and days and years on end: so that when conditions are less than optimal, we can still sing healthily and beautifully. I promise you that if you continue to work hard at your techniqiue under the guidance of a good teacher, you will reach that point someday.
4. This is so important that it bears repeating: if you are unsure about what to do with your voice, talk to your teacher or coach. They know your voice and will be able to give you an informed recommendation. They can also show you how to alter your warm-up routine, if necessary.
So, practice, practice, practice! And stay healthy!