Protozoa and Larger Parasites often escape disinfection processes, and physical removal by mechanical filtration is often the most cost-effective remedy. Boiling for one minute will kill everything except bacterial spores, but that is the most costly approach. No chemical disinfectant has been found reliable, and standard ultra-violet systems are ineffective. It happens that the parasite most difficult to kill or remove is also one of the most prevalent – Cryptosporidium oocysts may be as small as 3 microns in diameter, but most are 4-7 um in size. They can be found in virtually all surface raw water supplies worldwide, and no municipal or regional waterworks anywhere can guarantee killing or removing them all. Most strive for 99.9% reduction using multiple process barriers, but many do fine-filter with 99.9% efficiency (or better) for reduction of 1-micron particles at the point of use. So far, Cryptosporidium is the smallest, and 1-micron filtration is good enough. All other known waterborne parasites – Giardia, Entamoeba, and Cyclospora cysts, various round worms, tapeworms, flukes, and their eggs, Schistosoma larvae, etc. – are much larger.