Boiling is also an optional water treatment at household level. Boiling is a simple way of killing any ova (eggs), cysts, bacteria and viruses present in contaminated water. Water should be heated until large bubbles are continuously coming to the surface of the water. The disadvantage of boiling as a treatment method is that it requires large amounts of fuel, so cost may prevent people from using this method. Also, boiling may give an unpleasant taste to the water, which may be unacceptable, and very hot water can cause accidents in the home. Boiled water can become recontaminated once it has cooled.