A Covid outbreak has hit five adult wards at a north Wales hospital.
Ysbyty Gwynedd, in Bangor, said 49 of the 75 coronavirus patients being treated are linked to the outbreak.
The hospital's medical director Dr Karen Mottart said testing had begun for staff and patients.
"As we continue with a programme of additional testing, we expect to find more cases in patients and staff who are not currently symptomatic," she said.
Dr Mottart added: "This approach will help us build an accurate picture of the current situation, enabling us to safely manage patients and prevent any unknown potential transmission."
She said new variants are more transmissible and more patients have recently been admitted to the hospital than at any time during the pandemic.
"A significant risk is that a patient can test negative for Covid-19 while the virus is in its incubation period, and then test positive after being admitted to hospital," Dr Mottart added.
"All hospitals face the challenge of balancing the risk of infection while providing care for those who need it."
She asked people to help reduce the risk of transmission by only visiting hospitals when it is "absolutely necessary".
The area has some of the highest case rates in Wales - with 94.7 per 100,000 people in Gwynedd and 112.8 per 100,000 in neighbouring Anglesey.
Plaid Cymru's health spokesman Rhun ap Iorwerth said he was "anxious to hear" about the outbreak.
The MS for Anglesey added: "It is yet another demonstration of the contagious nature of the virus around us and the need to take firm measures to bring down numbers.
"It also shows how easily pressure can mount on health and care services that are already under great strain."