Actually, this is about right. About 10-15% of people are immune. Another half never get anything beyong minor symptoms.
Interesting, I heard somwhere that more beaver get Gardia from people than t'other way round.
I would not worry too much. A good segment of the population may or may not have it...hence good sanitary habits are always needed.
Unless you know, with a high probability, that you have it, I would not treat out of hand. Just my 2 cents...