you make a real good point here. good thinking. i still, not very often, mind you (i try, sometimes, to act like an adult) eat "virgin" snow.
there are no pathogens, TMK (to my knowledge), that i can think of that are found in "virgin" snow. how they would get into pure "virgin" snow, i can't even imagine. if you're at all concerned, dig down a ways and you'll obviously find snow not exposed, except for while falling and a short time afterwards, to whatever else might be in the air - if anything (these aren't the types of viruses, i.e. airborne pathogens, we're normally concerned with anyways). this snow also has never been in contact with the ground or any living thing. also, not too many birds flyin' about during a snow storm, right?!
[note: BTW, HTST (hi-temp, short time) pasteurization of milk is performed at ~162deg F for ~16-20sec. this is sufficient to kill bacterial and viral enteric pathogens - acc. to the FDA. also, all "bugs" in the milk get ~5log (IIRC), reduction fr/HTST pasteurization. this is a common method of pasteurization used by dairies. i only mention this to show that, for snow at least, boiling is not necessary to obtain pure drinking water.]
i gotta' believe that it's 'ok' to eat/drink this snow/water w/o purifying. why? kids all over the world would be comin' down w/all manner of afflictions, diseases, and plagues. many of us would never have made it into adulthood with all of the snow we ate. i know, not a very scientific method, but w/so much anecdotal evidence to the contrary, how can we believe pure "virgin" snow can make one sick? just think of kids growing up for past several millenia as one giant field experiment in snow eating. our observation of the results of this experiment would be that kids can eat pure "virgin" snow w/impunity - and this snow isn't even heated to make drinking water. i think we would be safe in extrapolating these observations to adults.
as a kid growing up in 50's and early 60's, the only popular concern (prob. a mother's phobia) we heard was radioactive fallout from the "bomb" tests (in the NE, we live on the receiving end of the jet stream/prevailing winds). [i ate a lot of snow - guess that explains quite a few things...!!!]
oh...and...as mom always warned, "Don't eat the yellow snow!!".