Parasites are huge creature as are cysts. ALMOST ALL current filters will remove them. Nuff said.. Trace dissolved contaminants are another story, but are no big deal, IO.
AS long as you use some type of commercial filter, AND sanitize your onboard water, you have almost no chance of getting sick.
We use an inline water filter also but I do think its important to disinfect the fresh water supply faucet to your camper before hooking up your fresh water hose. You never know what the last thing the previous campers touched before turning off the water supply.