Eleven ambulances were sent to a care home in Cornwall at the weekend after a coronavirus outbreak.
Coronavirus rapidly spread to 25 residents at Trengrouse care home in Helston and many staff were off sick or self-isolating despite best efforts to prevent the spread, bosses said.
Three residents were taken to hospital. Others needed oxygen and are being looked after at the home.
Bosses said staff and residents were being "tested very regularly".
The multiple ambulances were called on Saturday night, but manager Anne Thomas said "we'll never know how it came into the home".
She said the home had "managed to go several months without any outbreak at all ... [but] it still seems to be breaking through our best endeavours", particularly the "new very virulent strain" of the virus.
The outbreak meant a "fair number" of staff were off sick and self-isolating.
She added: "However careful we are, unfortunately, there's no way of knowing how it got into the home".
Some of those who contracted it "had no symptoms at all", she said.
The South Western Ambulance Service NHS Foundation Trust said it received "multiple calls" to the home and sent "a significant amount of resources to provide care to patients at this location".
It added: "We allocate our resources according to clinical need and it's not unusual to send multiple resources to incidents."
Cornwall Care runs 16 homes in the county. Residents have been vaccinated at six of them.