An 80-year-old woman resident at the home in Hillesheim, Rhineland, died on Saturday in hospital after becoming ill towards the end of March.
She had shown the same symptoms that 12 other residents had developed – they complained of nausea, vomiting and severe breathing problems.
Problems began around three weeks ago, since when eight people have now died.
Of the post mortems conducted so far, three have shown cause of death to be natural, but doctors are at a loss to explain their symptoms.
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In four cases the cause of death has not been established and inflammatory changes were seen in their lungs.