Up to 30 young people in Plymouth could be infected with coronavirus having returned from holiday in Greece, local health officials say.
The city's public health team said the group, aged 18 and 19, returned from the island of Zante last week and so far 11 have tested positive.
Many of them had no, or "very minor", symptoms of the virus, they added.
Greece is not currently on the list of countries with quarantine restrictions for UK travellers.
Plymouth's director for public health Ruth Harrell said her team were working alongside national systems to contact and trace those thought to have been affected.
Some who were not showing symptoms "carried on as normal" until they became aware of the risk, including going on a "night out in Plymouth's bars and restaurants", she added.
She said: "While we are still below the point of triggering a lockdown, this incident just goes to show how easily life can change."