The head of the French health service, Lucien Abenhaim, said people should be on their guard for the next eight weeks - the maximum incubation time for listeria infection.

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The incubation period, the time from when you're first exposed to the time you express symptoms, is usually 1-5 days, but can be as long as 60 days.

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SARS, says Longini, was contained partly because it has a relatively long incubation period of six days, giving authorities more time to respond.

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