A corona infection was first diagnosed in the Greek refugee camp Moria on Lesbos.
Aid organizations have long feared that the corona virus would emerge, because many people live on top of each other and medical facilities are limited.
The infected refugee is a 40-year-old man from Somalia.
He has been placed in isolation and the Greek authorities are trying to find out who he has been in contact with.
Moria, the largest refugee camps on the Greek islands, is currently home to thirteen thousand refugees, while there is actually room for only three thousand people, Oxfam Novib said in a response to the news.
"Without intervention, this outbreak could lead to the death of hundreds of already very weakened people. It is imperative that people be evacuated from the island, including the most vulnerable," the aid agency said.
Since March, refugees arriving on Lesvos are first quarantined.
How the virus could still show up in Moria is still unclear.
Lesbos is located near the Turkish coast.
In recent years, thousands of asylum seekers have arrived trying to reach Europe.
'Corona in packed Greek refugee camp means disaster'