Dozens of people have been feeling violently ill since a drum and bass night at Falmouth Rugby Club on Saturday night.
People who attended a christening at the same venue the following day have also been reporting feeling sick and having diarrhoea.
Public Health England (PHE) said it is likely to be norovirus with up to 40 people who attended the club infected.
The venue has voluntarily shut and will be conducting a deep clean.
PHE is investigating with Cornwall Council.
The chairman of Falmouth Rugby Club James Instance said: "We have closed the club to the public pending results of their investigations and shortly will be undertaking a deep commercial clean of the premises."
Therapy DnB, who organised the sold out event, had been advising people who attended to see a doctor, and have been posting regular updates on Facebook.
Hedex, one of the DJ's at the event, said on Twitter on Monday: "I am so ill I am considering to start writing my will."
Dr Sarah Harrison, consultant in health protection for PHE South West, said: "Norovirus is highly infectious.
"PHE is working with environmental health officers from the council to ensure that appropriate precautions and procedures are in place."
The latest advice from PHE is that anyone with suspected norovirus should drink plenty of water and avoid preparing food for, or direct contact with, others until 48 hours after the symptoms have stopped.