Have you noticed that your voice doesn't always sound the same? Some days the lower part of your voice is freer than the upper registers, or vice versa. Somedays it may sound dry or rough, some days light and breezy. You might think "I was sounding so good yesterday and now I can't get there!" This isn't always due to technique. Everyone has on days and off days with their voice, even professionals. Our body is our instrument and we are human; we get sick, we indulge in delicious food, we drink, we get stressed...All of this can contribute to how our voice sounds and how our throat feels. And the more we are aware of how these things effect our voice, the more we can do to take care of it.
Being Sick: Drink plenty of fluids to stay hydrated. If you have a sore throat, tea with honey is soothing for the throat. It doesn't have any effect on the vocal folds themselves but can comfort the muscles in the larynx. Some products out there are great like Throat Coat tea and Singers Saving Grace(a throat spray), both of which you can find at a natural foods store. You can still sing when you're sick but it depends on what your sick with and how it's effecting your throat. If there is any swelling in the throat don't sing, and speak as little as you need. But a simple cold usually doesn't effect the throat. With the flu/stomach bug, wait to sing. And if you lose your voice, try not to speak at all, but especially DO NOT whisper. That is actually very bad for the throat. And trust your instinct. If it feels bad, stop.
Food: In my experience, there is no black and white list of foods that effect the voice. Some foods can overproduce mucus, or cause a flare of reflux if you have that. But really noticing if there are certain foods that effect your voice comes from you checking in with yourself. Does your vocal quality always change the day after you eat pizza or ice cream? Or eat an orange? Or drink wine? Does it feel like singing takes more work in your throat? You can ask yourself, what have I been eating/drinking? As you note these foods, notice if there is a trend and become smarter about what you decide to eat and drink before an audition or an important performance.
Water: When I learned this it changed my life: Science says it can take up 2 days to rehydrate. That means if you want to hydrate and make sure your vocal folds are lubricated before a lesson, audition or a performance, it doesn't matter what you drink that day. You can drink 3 bottles of water and it still won't hydrate you. Start being aware of your water intake a few days before. In fact, the best thing to do is to always drink enough water, every day! But sometimes we forget.
Stress: We are human, so we get stressed out, sometimes have anxiety, emotional ups and downs. This can effect your voice in different ways. And this is where singing can help you and be your friend. Stress and anxiety often effects the breath, causing shorter inhales, tight and high in the chest. Slowing down and connecting to your breath, allowing for a deeper breath as you practice can help. Letting yourself HAVE your inhales. I often talk about the importance of the inhale, that singing isn't just about getting to the notes. Slow down a vocal exercise and simplify it to let your breath and your sound have more space, and let your mind slow down and give yourself a break. Even if you only sing one note but do it with this attention, I call that success. It may take more than a few minutes for your body and voice to relax and respond, so be patient with the process.
Being a singer can feel like a 24/7 job. And that's ok! As you begin to notice what effects your voice, you will begin to have a new awareness and relationship towards your body and your singing. And the care you take of yourself as a singer is good for you as a human.