Staff at a hospital nursery are being tested for the E.coli virus.
It is thought three toddlers have already contracted the potentially deadly bug from the Hilltop Nursery at Brighton General Hospital.
A three-year-old child has tested positive for E.coli and is being treated in a London hospital.
A two-year-old and an 18-month-old are also suspected of contracting the disease and are being cared for at home.
The nursery cares for the children of hospital staff working at four NHS trusts.
It looks after 40 children aged between six months and school age.
The nursery has been closed all week for "deep cleaning".
On Friday, mops and buckets, vacuum cleaners and other cleaning equipment could be seen at the nursery.
A notice said the "deep cleaning" had been completed but the nursery would remain closed.
A spokeswoman for South Downs NHS Trust said all staff would have to be tested before returning to work.
E.coli is a potentially lethal virus which causes diarrhoea, abdominal cramps and blood in stools.
Children and the elderly are particularly prone to catching the virus.
It is particularly dangerous to young children because their bodies are less well equipped to deal with a sudden loss of a substantial amount of fluids.
The nursery has been closed since Friday, September 8.
The incubation period for the E.coli virus is two weeks, parents are being kept up-to-date by phone.
Three years ago the nursery had a £100,000 refit so staff could care for babies under 12 months old.
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