At the moment, quite a number of my students are going down with colds, sore throats, coughs etc. In my experience, the sore throats aren't necessarily a virus but contributable to other things like tiredness, shouting(!), singing too loudly, singing for too long or at inappropriate times (like in the middle of the night!), food intolerances, bowel problems. The way in which somebody speaks can also lead to a sore throat if they naturally have a higher voice and instead lower their voice either intentionally or unintentionally as this can strain the voice.
The trick, as a singer, is obviously to try to avoid infections. What I do may seem extreme to some, but has successfully helped me to avoid getting a cold or other upper respiratory virus for years. Whenever out, I just don't touch things like door handles or whatever and am vigilant about using gloves, cardigans etc to do this for me. I also carry baby wipes with me so that I can wipe my hands when I need to, as even using a keypad on a card reader can of course carry lots of unwelcome germs. When I return home, I wash my hands immediately.
As a self-employed person, I'm clearly motivated not to get ill as if I do, I then don't get paid (although I appreciate that if you work in a school where children just sneeze over you, then you have no control over that, however, for most of us, there is a lot of control we can take back and I think it's worth it as it's horrible having colds or flu-like vliruses).
If you do get a virus, we have learned how to turn this around very quickly: colloidal silver (5 sprays, 5 times per day - available from good health food shops), echinacea (full herb tincture) 15 - 20 drops 3 times per day, zinc, vitamin C - 1000 - 2000 mg but it must be the slow release type and sambuccol - which actually helps to increase red blood cells and tastes wonderful. Lots of warm liquids etc.
Exercise of course will also help to keep your body fit, but don't over-exercise, think about what you need to be fit for!