People are being advised to stay away from some hospitals in south Wales unless necessary due to a "significant increase" in flu cases.
The warning has been issued by health board bosses covering hospitals in Swansea, Port Talbot and Bridgend.
Abertawe Bro Morgannwg health board, which covers the region, said the move was to avoid spreading the virus.
Consultations for flu have doubled from the last week of 2017 to the first week in 2018, Public Health Wales has said.
There were 37.7 consultations per 100,000 people from 1-7 January, compared to 16.7 consultations per 100,000 in the last seven days of 2017.
An ABMU health board spokesman said: "We now are seeing a significant increase in confirmed flu cases in our hospitals.
"Please avoid visiting unless you really need to, and if you have a flu-like illness please stay away from our wards in case the infection is spread to patients."
The health board manages Singleton and Morriston hospitals in Swansea, Neath Port Talbot Hospital in Port Talbot and the Princess of Wales Hospital in Bridgend.
It comes as 68 emergency doctors, including five from Wales, have signed a letter to the prime minister spelling out the danger patients are facing this winter, amid claims safety is being compromised by "intolerable" conditions.
Public Health England also issued a warning on Thursday that hospitals in England are seeing very high rates of patients with flu.
Last week, ABMU health board said it had to cancel "most" routine planned surgeries.
On Wednesday, Health Secretary Vaughan Gething told AMs there were more "difficult days" ahead as winter pressures continued to affect the health service.