Hving ridden on the same merry-go-round myself I try not to get trapped in the futile pursuit of a definitive answer to the question of whether my source water is contaminated--none exists. The shark attack metaphor is a canard (hey, that's a mixed metaphor right there) because if you've been attacked by a dang shark you dang well know it. By contrast, if you've been exposed to giardia (or any of dozens of waterborne pathogens) you 1. don't know it at the time, 2. may remain asymptomatic and never know it, 3. may become ill after a time but never receive a definitive diagnosis or, if diagnosed, are unlikely to know where or how you contracted it.
So the proper science to apply is not microbiology, it's risk assessment—instead of summoning a microbiologist, seek an actuary. The vast array of portable treatment technology exists gives us the means to reduce our exposure to waterborne pathogens, relatively easily. The decision rests with us whether to treat. If you think water is safe, you collect and drink it. If you think the water could be "contaminated," then you filter/treat. The cautionary tale most folks with a lot of backcountry time in have, is the unfortunate shock of discoving their presumably pristine water actually has a stomach-churning contamination source (such as an upstream animal carcass or cattle bog) in seemingly safe waters.
Today's water treatment is mostly quite easy and adds a layer of protection. To use it or not is similar to clicking in a car's seatbelt--there it is, why not use it? One argument I've yet to hear is "If I do that I'll feel TOO safe."
As a sidebar, we didn't even know giardia was a pathogen until the 20th century, yet it was discovered by Van Leeuwenhoek in the 17th. Cryptosporidium wasn't commonly discussed until major, citywide outbreaks sickened thousands. To drag sharks back in, they travel and a safe beach today may be a great white snackbar tomorrow. Likewise, a safe creek today may host a dead moose next spring. Lacking fieldborne test equipment, how does one manage their personal risk making their treatment decision?