Wiltshire Police said the area surrounding Prezzo restaurant has been closed while they work to establish what caused the “medical incident” involving a man and a woman on Sunday.
The ambulance service alerted detectives at about 6.45pm. Firefighters were also in attendance.
Witness Sam Proudfoot, 16, said he saw a person in a hazardous material suit go between the restaurant and the ambulance.
“I’ve been told two people were taken ill in there,” he said.
“There’s a man in a full white body suit with a mask to his mouth going in and out of the back of the ambulance and the restaurant.”
Wiltshire Police said: “Police were called by the ambulance service to Prezzo, High Street, Salisbury at 6.45pm today following a medical incident involving two people.
“As a precautionary measure, the restaurant and surrounding roads have been cordoned off while officers attend the scene and establish the circumstances surrounding what has led them to fall ill.”