I am a MSc. student at the University of Calgary
working on detecting giardia and cryptosporidium.
As far at the questions you are asking...I would
ONLY trust some sort of <5um filter of some sort
or boiling. Boiling does reduce the viability of
giardia and crypto by several log factors and
should ensure safe drinking. Iodine is a little
more iffy....giardia will be killed, crypto may
not be...its a tough bugger. I personally use a
backpacking water filter or boiling when i am out
and about in the rockies where giardia is a prob.
As far as the time for boiling....i am not sure
but the longer you boil the more likely all the
nasties will be dead. I could pull out alot of
references on viability studies at various
temperatures but none that are specifically
applied to wilderness...if you really have to know
i would try to contact someone from backpacker
magazine or the like and ask them for a source for
info on this topic.