• The concentration of the organism in the water;
  
• The virulence of the organism;
  
• The per capita intake of the infected water;
  
• The infective dose of the organism;
  
• The susceptibility of the individuals;
  
• How widespread the organism  has infected the community (which determines the number of organisms being excreted).
  

The level of immunity of the community to the particular organism is also related to the occurrence of infection. If  the community is generally resistant to the organism due to prior exposure, then only visitors who frequently drink the water or food prepared using contaminated water will become ill.

Barriers to the Transmission of Pathogenic Microorganisms
Water used for drinking or food preparation should not contain organisms capable of causing disease. Microorganisms, including pathogens, may enter water supplies at every stage of the collection and distribution cycle. Clearly water must be protected from faecal contamination, and once the water is in the system, natural processes and die-off will limit the transmission of pathogenic microorganisms. However, these processes cannot be relied upon to ensure that the desired water quality is maintained throughout the distribution system. Ensuring the microbial safety entails a wide-ranging program of protection, treatment and monitoring, with barriers to the entry and transmission of pathogens throughout the system. The barriers should include at least most of the following measures, note that the figures in brackets after a group shows the percentage removal of faecal coliforms which can be achieved by the barrier mechanism:

• The water sources should be protected from contamination by human and animal faeces.
  
• The water should be pre-treated, for example by detention and settling in reservoirs for sufficient time to allow bacteria to die off, 3-4 weeks (>80%).
  
• Water storages should be protected.
  
• Coagulation, settling and filtration should be carried out (>80%).
  
• The water should be disinfected before it enters the distribution system (>99%).
  
• An adequate disinfection residual should be maintained throughout the distribution system (>99%).
  
• The distribution system should be secured against possible re-contamination. This entails ensuring the integrity of the pipe system, vermin-proofing water tanks, and preventing back-flow.
  

Microbial Indicator Organisms
Monitoring for specific bacterial, viral and protozoan pathogens is usually complex, expensive, and time consuming and may fail to detect their presence. In some cases the results of these analyses may take weeks. In monitoring for microbiological quality, reliance is therefore placed on relatively rapid and simple tests for the presence of indicator organisms. An effective indicator organism for inferring the presence of pathogenic microorganisms in drinking water should:

• Always be present when pathogens of like origin are present;
  
• Be present in relatively large numbers so that they are easily detected after considerable sample dilution;
  
• Not be present in the absence of contamination;
  
• Be easy and quick to detect;
  
• Survive in water as long as water-borne pathogens; and
  
• Be of similar sensitivity to disinfection as pathogens