Although some species of Legionella bacteria can be found in the soil, most species live in water that is stagnant and wherein such bacteria survive under a wide range of temperatures.  According to the Centers for Disease Control and Prevention, USA, between 8,000 and 18,000 people are hospitalized with Legionnaires disease each year.  It is of great public concern as its fatality rate during an outbreak ranges from 5% to 30% in those who contract the disease.  Actively managing the risk of Legionella in water systems is more cost effective than responding to an outbreak.  Outbreaks of Legionella pneumophila can stem from showers and potable water systems.  As water from such sources aerosolized, individuals can inhale the Legionella containing droplets and the organism is aspirated into the lungs. 
The formation and multiplication of such Legionella bacteria is not only promoted by the temperature in the customary hot water systems, but also by the fact that dead spaces are present in such water distribution systems in which deposits and sediment formation can arise, and typically in the bottom zone of water heater tanks.  Deposits therein can represent a culture medium for bacteria proliferation. 
Most electric water heaters for domestic use have their water tank constructed with a dome shaped bottom wall.  Such dome-shaped bottom walls form a surrounding cavitated zone about the dome-shaped wall where sediments deposit can gather and where water is less agitated.  This cavitated zone is spaced from the bottom heating element and thus water therein is less hot which can create an ideal location for bacterial growth.  Should the bottom element fail, then the water temperature at the bottom of the tank will drop.  It as also been determined by research that the Legionella bacteria does not survive at temperatures above 140 degrees F. When hot water is not drawn from a water heater, the water inside the tank becomes stagnant and the water temperature stratifies with the cooler temperature being at the bottom region of the tank.  Water within the cavitated zone below the bottom element of the tank can fall to about 85 to 105 degrees F. which is favorable to bacteria growth.  It is said to be difficult to increase the temperature at the bottom of the tank when there is not water consumption.  The solution to the problem is to raise the temperature in the bottom end of the tank.  However, lowering the bottom element to place it close to the bottom wall of the tank has not proven to be a viable solution. 
Reference is made to U.S. Pat.  Nos.  4,940,024; 5,168,546 and 5,808,277 which disclose various methods and apparatus to prevent bacteria proliferation in electric water heaters.  One method teaches adding a heating element in the form of a belt or patch on the outside of the tank against the bottom end of the outer sidewall of the tank to heat the water at the bottom end of the tank to a temperature preferably above 55 degrees C. (130 degrees F.).  Accordingly, this proposed solution provides an extra heating element in the form of one or more electrical patch heaters located in an area which is usually filled with insulating foam material and not practical to access should it fail and require replacement or repair.  Providing a large access panel for access to such elements is not practical as the expandable foam would cover the elements and if a large foam dam is used then there will be considerable heat loss in that region.  It is not a practical solution and it is also costly and consumes more electricity.  In U.S. Pat.  No.  5,808,277 a third element is added into the tank to periodically raise the water temperature at the bottom of the tank beyond the pre-set consumption temperature, to a sanitizing temperature to destroy bacteria.  This is also a costly proposition.  U.S. Pat.  No.  4,940,024 discloses a method of directing the cold water flow of all consumed drinking or domestically used water through the lower region of the tank wherein there is no stagnant water and wherein no deposits can be formed for bacteria growth.  Accordingly, the lower region of the tank is continuously flushed with fresh water.  This is a costly solution requiring a new tank design and cold water conduit network and therefore not a viable solution.