Single cell animals bathe in UV and get a tan. This includes giardia.
Where I learned about water quality was firing a pair of B&W boilers while going to college. Diatomaceous earth, ion exchange, etc. We filched all of the books out of the abandoned Platformer unit and they became our library. In between maintenance, blowdowns, taking readings, and worrying what that noise was, we studied.
Mendocino County CA. Three instances, two hundred miles apart, drinking well water for a month or longer, my hands swelled to double size. Medical community was baffled. No change in diet, or habits. The third time, I went to the store and purchased distilled water. Hands returned to normal size with a lot of peeling skin. Friend got angry when I suggested it may be something in the water. Indignant as hell as a matter of fact. Went back to drinking (good tasting) well water and hands swelled back up. The odds against all this being coincidental is laughable. Remember, three locations. Three wells. Two hundred road miles separated the wells. A person tends to get touchy about water quality after experiencing "catcher's mitt" size hands. No, it hasn't happened before or since.