The number of people in Ilkley suffering from a gastro-intestinal illness caused by the giardia lamblia parasite has risen to 46.
Dr Martin Schweiger, consultant in communicable disease control at the West Yorkshire unit of the Health Protection Agency, said: "We are continuing to encourage people to contact their GP if they feel unwell.
"If someone has already tested negative for giardia but their symptoms persist, then they should go back to their GP to be tested again.
"We have alerted family doctors in the local area to look out for people showing signs of the illness - which are diarrhoea, gas or flatulence, indigestion, nausea and stomach cramps."
The Health Protection Agency and Bradford Council's environmental health department are continuing to investigate the cause and source of the outbreak.
The Saffron restaurant in Station Plaza, Ilkley, is the focus for these investigations.
The water supply to Ilkley and several other environmental sources of giardia lamblia have now been excluded.
Dr Schweiger said: "The spread of gastro-intestinal infections can be limited by ensuring thorough handwashing."
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