So much depends on where you are when you start a water fast, of course. Only you, the faster, can know how close to well or sick you are. But this is truth: you only 'catch' things (like flu and cold) when you are already congested, and out of balance. Meaning, what we call catching something is actually the body, in its wisdom, (which was already low-level sick) making itself well! by throwing off the crud that had been building up inside. That we "feel sick" while this housecleaning is going on is only a reflection of how congested we are (or rather, have been.) There's nothing really to fear, as in, something 'out there' attacking.
If you go through a downswing, you just ride it out. It goes much faster, with water only. Don't think/worry of catching something new--Rather see it all as release of the old. Accept it and let go. My last Water Fast I only wanted hot water the whole time. It can be like the best medicine around.