Low temperatures stop the growth and multiplication of bacteria but they permit viruses to pass through. If a water-borne virus epidemic, such as infective jaundice, is raging, these filters are ineffective. Boiling filtered water is safe. Electric water purifiers are probably effective but need constant maintenance which is costly and not always available. Candle filters are reasonable effective to filter off bacteria and suspended impurities but they permit viruses to pass through. Animal vectors, the carriers of infection such as mosquitoes and housefly are eliminated by chemical sprays of insecticides, or their entry is prevented by wire-gauze shuttering or mosquito nets.
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