While no one can debate the benefits of good hygiene, bacteria, viruses and parasites have to come from somewhere. Individuals infected with Giardia sp. are picking it up from somewhere; most likely source is the water.

It is possible a partner is infected and passing the Giardia sp. along to others due to poor hygiene via fecal contamination - oral route of exposure. However, I would think the water is still the most likely culprit, despite the test results, which may not be sensitive enough to detect low-level contamination.

Can you find and cite the studies?

Pete