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Other aspects of microbiological quality As noted above, water borne disease is not exclusively transmitted by the faecal-oral route, although this route of disease transmission is of overwhelming importance globally. Some other microbiological aspects of importance are as follows: Opportunistic and other water-associated pathogens Opportunistic pathogens are naturally present in the environment and normally present no risk to human health. They are able to cause disease in people with impaired local or general immune defences. These people include the elderly and the very young; persons with extensive burns; persons undergoing immuno-suppressive therapy (such as following transplant surgery) and those with immuno deficiency-related diseases (such as AIDS). Examples of opportunistic pathogens of this type include Pseudomonas aeruginosa, certain species of Flavobacterium, Acinetobacter, Klebsiella, Serratia, Aeromonas and some ‘slow growing’ mycobacteria. WHO SEMINAR PACK FOR DRINKING-WATER QUALITY 6 Inhalation of water containing certain infectious agents may also cause disease. This is the case with, for example, Legionella spp (Legionnaire’s disease) and Naeglaria fowleri (an occasional cause of primary amoebic meningoencephalitis). Cyanobacterial Toxins Some cyanobacteria (‘blue-green algae’) are capable of producing toxins, including hepatotoxins, neurotoxins and lipopolysaccharides. Few epidemiological studies have been undertaken and little information is available regarding the true importance of this problem. Where blooms of cyanobacteria occur in lakes and reservoirs used for drinking-water supply a potential risk to health exists and therefore impounded surface waters used for drinking-water supply should be protected from contamination with nutrients.