I typically use Katadyn Micropur tablets. These will kill all bacteria and virus within about 15 minutes. According to the directions, four hours exposure is required to kill the protozoa Giardia and Cryptosporidium. A filter is a better alternative if you are concerned with protozoa. But, I have never heard of someone coming down with anything after using the Micropur. The only problem with Micropur is the swimming pool water taste they give the water.