Drinking water may contain all types of microbes-viruses, algae, molds, bacteria, and parasites-but they are not all necessarily dangerous to health. Some just cause bad taste and odor or clog filters. Algae and molds seldom have any health significance, but all products and claims relating to the other three microbial types are watched carefully by public health officials. At present, Cyst Reduction is the only permitted microbiological reduction claim, unless bacteria, viruses, and cysts are all killed/removed simultaneously. The requirement for that is 3-109 reduction (99.9%) for Cryptosporidium oocysts, 4-log reduction (99.99%) of two kinds of virus, and 6-log reduction (99.9999%) of Klebsiella terrigena bacteria, which is an environmental coliform. Any product that can document all three abilities can be labeled a “microbiological purifier.”