Water heating by a domestic water heater. Two conflicting safety issues affect water heater temperature—the risk of scalding from excessively hot water greater than , and the risk of incubating bacteria colonies, particularly "Legionella", in water that is not hot enough to kill them. Both risks are potentially life-threatening and are balanced by setting the water heater's thermostat to . The European Guidelines for Control and Prevention of Travel Associated Legionnaires’ Disease recommend that hot water should be stored at and distributed so that a temperature of at least and preferably is achieved within one minute at points of use. If there
Rare disease assumption odds ratio (OR) is equal to 16 and the relative risk (RR) is equal to 8.65. Although the prevalence in our example equals 10% it is very difficult to apply the rare disease assumption because OR and RR can hardly be considered to be approximately the same. However, in this example the disease is not particularly "rare"; a 10% prevalence value means 1 in 10 people would have it. As the prevalence drops lower and lower, OR approaches the RR much more closely. This is one of the most problematic aspects of the rare disease assumption, since there is no