Health officials are investigating an outbreak of an eye infection in Rhondda Cynon Taf.
Public Health Wales have identified 25 cases of viral conjunctivitis in people associated with the Tegfan Resource Centre Care Home, Aberdare.
It is "highly infectious", causing a red watery eye with discomfort and blurred vision, lasting two weeks.
Letters are being sent to schools, GPs, pharmacists and opticians in the area. The home has declined to comment.
Dr Gwen Lowe, consultant in communicable disease control and chair of the outbreak control team, said: "Strict infection control procedures have been put in place throughout the home and all unnecessary visits have been stopped."
Dr Lowe added that the control measures put in place are designed to limit the spread of the infection.
"However, we are expecting to see more cases as this is a highly infectious illness," she said.
Dr Lowe said Rhondda Cynon Taf County Borough Council, Cwm Taf Health Board and local eye specialists are helping to limit the spread of the infection.
Antibiotic eye drops do not cure the illness but they may help prevent secondary eye infections.
Public Health Wales said it will continue to monitor the situation.