Seven patients including five children under the age of 8, have been infected with E. coli O157 in recent weeks. At least one is known to have been hospitalised.
The Health Protection Agency (HPA) is working with Birmingham City Council to investigate the outbreak at Sutton Park in Sutton Coldfield.
The HPA said the strain of the bacteria was often associated with contact with rural environments that may have been contaminated by animal faeces.
The advice being given is to wash your hands, especially after contact with animals, after going to the toilet and immediately before eating.
Cyclists and walkers are also being advised to wash their tyres, footwear and hands after visiting Sutton Park as an extra precaution.
Sutton Park, covers around 2,400 acres and is thought to attract around two million visitors annually.
Designated a National Nature Reserve, the former royal deer park is included in English Heritage's list of historic parks and gardens, and includes sites of archaeological interest.