Even better, use a 0.5 micron filter to get rid of cryptosporidium cysts and other big stuff.
Chlorine dioxide kills viruses and bacteria quickly, but the crypto cysts are very tough. They can survive in undiluted bleach for 24 hours.
In clear 20ºC water, chlorine dioxide kills everything, including the cysts in 15 minutes. In cloudy, cold water, it can take four hours. The cysts can be as small as 4 microns, so the 0.5 micron (absolute) filter takes care of them.
I use a $3 filter from Duda Diesel, designed for filtering biodiesel fuel.
I learned about this filter from Roleigh Martin, a very active hiker on the John Muir Trail.