There have been 267 cases of the bug since May 11 across four areas of England, more than double the normal rate.
The parasite Cryptosporidium is normally associated with contained swimming pools, lakes and salad foods but no source has yet been identified in this outbreak.
People in the North East, Yorkshire, West and East Midlands have been infected.
Usually outbreaks remain isolated and the source of the infection is quickly identified.
The Health Protection Agency has investigated the drinking water supply and it is thought this is an unlikely cause of the outbreak.
The organism can live in soil, food, water and on surfaces contaminated by human or animal faeces.
The infection known as cryptosporidiosis causes diarrhoea, stomach pains, dehydration, weight loss and fever which can least a month or more. Symptoms can be severe in the very young and the elderly, however most people affected in the outbreak so far are adults aged between 15 and 45.
A 'handful' of people have been admitted to hospital and have now recovered, a spokesman for the HPA said.
Dr Stephen Morton, who is leading the investigation for the HPA, said: “It is usual to see an increase in cryptosporidiosis cases in the early summer, but, the increase is higher than we might expect so we are working with NHS partners, local Environmental Health Officers the Drinking Water Inspectorate and the Food Standards Agency to see if there is a common source of infection.
“GPs in affected areas have been informed about the recent rise in cases and asked to be vigilant to further possible cases. This is to ensure any further possible cases can be followed up as part of our investigations.
“If our investigations identify a common source, we will issue further health advice to the public as necessary.”
The dry spell followed by heavy rain could have caused irrigation systems to become contaminated but there is no evidence of that yet in this outbreak, a spokesman said.
Tap water was infected with cryptosporidium in 2008 in Northamptonshire when residents were told to boil their water.
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