Pupils at two south Wales primary schools have been offered vaccinations against hepatitis A, after five cases of the disease were identified.
Officials said they are investigating the cluster of viral infections in the Vale of Glamorgan.
Pupils in specific years at Victoria Primary School in Penarth and Palmerston Primary School in Barry have been offered jabs "as a precaution".
The disease is described as "unpleasant" but not usually serious.
"There is no indication that the infection has been contracted on either school premises, and both schools are co-operating fully with arrangements," said Dr Rhianwen Stiff, at Public Health Wales.
"Hepatitis A can be unpleasant, but it's not usually serious and most people make a full recovery within a couple of months. Children often only have a very mild illness or do not have symptoms at all.
"The virus can spread easily between young children, who will often not have symptoms but can still transmit infection to other people."
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Hepatitis A facts
An infection of the liver caused by the Hepatitis A virus
Anyone can be infected with Hepatitis A. A large number of people who become infected contract the virus abroad
The virus is excreted in faeces and can be passed onto people when something contaminated (e.g. food, water) is put in the mouth
Tiredness, general aches and pains, headaches and fever are among the symptoms, as well as loss of appetite, nausea or vomiting, abdominal pains, jaundice, very dark urine and itchy skin
Complications are rare. Most people make a full recovery within two months. The illness can be more severe in people over 50
Anyone infected needs to be assessed by a doctor. There is no specific treatment. Alcohol should be avoided
Source: Public Health Wales
Dr Stiff said the best advice to prevent the spread of the illness was good hand-washing hygiene.
She said anyone who is concerned about the illness in the area, or is displaying symptoms, should contact their family doctor or NHS Wales Direct.