Vaccinations against hepatitis A are being offered to people who have been in close contact with a woman diagnosed with the infection in Merthyr Tydfil.
About 100 clients and staff at the Llysfaen Respite Centre and Sandbrook Day Centre are being offered the jabs.
The woman was diagnosed on Tuesday after catching the disease abroad. She is said to be recovering.
The National Public Health Service for Wales (NPHSW) said the risk of transmission was small.
Hepatitis A is a liver disease caused by the hepatitis A virus, which is passed from person to person by eating contaminated food or drinking water.
"There are currently no signs of further cases of the infection and the risk of transmission is very small"
Symptoms of the infection include tiredness, general aches and pains, headaches and fever, a loss of appetite, nausea, vomiting and diarrhoea, jaundice and abdominal pain.
Hepatitis A does not have lasting effects and is rarely fatal in younger people, but according to the NPHSW the illness can be more severe and can result in death in nearly 2% of cases in people aged over 50.
About 100 people in Merthyr are being contacted with an offer of vaccination.
Gwen Lowe, consultant in communicable disease control for public health body the NPHSW, said: "There are currently no signs of further cases of the infection and the risk of transmission is very small.
"However, after a detailed assessment and taking expert advice, we have decided to recommend hepatitis A vaccine for people who have attended the Sandbrook Day Centre and Llysfaen Respite Centre as a precautionary measure."
^ Back to top | BBC Sport Home | BBC Homepage | Contact us | Help | ©
~RS~q~RS~~RS~z~RS~23~RS~)