Greece on Wednesday announced a first coronavirus infection in its largest migrant camp of Moria on the island of Lesbos, where nearly 13,000 asylum seekers live in unsanitary conditions.
"A 40-year-old Somali man has tested positive," a migration ministry source told AFP.
The man is a recognised refugee who had recently returned to the island from Athens, the source said.
More than 12,700 people live in Moria, which has a nominal capacity for fewer than 2,800.
Another infection in an island camp on the island of Chios, involving a 35-year-old man from Yemen, had been reported last month.
Several non-fatal coronavirus cases have surfaced in Greek camps on the mainland -- including 150 infections at a migrant hotel in the Peloponnese in April.
Camps on the islands are nominally under lockdown until September 15, with access severely restricted.
But in Moria, many asylum seekers sleep in tents in an olive grove outside the walls of the camp, where restrictions are harder to enforce.
Rights groups including the UN refugee agency have repeatedly urged Greece to reduce congestion at the camps.
New arrivals on the islands are quarantined in separate buildings to limit the risk of contaminating entire camps.
With 271 deaths from Covid-19, Greece has suffered less from the virus than other European countries, and so far nobody has died of the disease in a migrant camp.