The theory that cholera outbreaks sometimes occur because tsunamis wash bacteriophage viruses lethal to cholera bacteria out of the soil should be treated with caution (15 January, p 8).

Marine forms of Vibrio cholerae, the bacteria that cause cholera, are common in marine algae and plankton in most tropical waters. But if growth conditions are unfavourable they revert to a dormant form that is probably immune to attack by bacteriophage viruses. So phage numbers may be low despite the presence of their target bacterium.

After tsunamis, the mixture of seawater and sewage provides perfect conditions for the bacteria to reawaken, multiply and become infectious again, threatening human survivors exposed to contaminated water. The persistence of the bacteria in puddles and wells ensures that the risk of infection remains for months.

Bacteriophages will undoubtedly increase once the bacteria reactivate, but the cholera bacteria are already present in huge numbers in the tsunami, and cholera outbreaks can occur as soon as 24 hours after an incident.

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