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, adding that the patient is in good health.
More than 12,700 people live in Moria, which has a nominal capacity for fewer than 2,800.
The migration ministry said the camp would be sealed until September 15, with only "security personnel" granted access after temperature tests.
The Somali is a recognised refugee who had recently returned to the island, even though he had been allowed to permanently relocate to Athens in July, the ministry said in a statement.
A camp source said the refugee is a diabetic.
The refugee's contacts are now being traced and "extensive checks" are underway at the camp, the health ministry said in a statement.
Another infection in an island camp, on 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.
The Somali refugee was among those sleeping outdoors, the Moria camp source told AFP.
Many refugees have been unable to find lodgings and jobs after leaving the camps, with housing and cash benefits recently scaled back by the government.
Rights groups including the UN refugee agency have repeatedly urged Greece to reduce congestion at the camps.
On Lesbos, medical charity Doctors Without Borders (MSF) in July said it had been forced to shut down a coronavirus isolation centre serving the local camp over a planning dispute.
The group has criticised the camp lockdown, which has been repeatedly renewed since March, as "discriminatory" and "counter-productive".
The migration ministry on Wednesday said a new isolation unit for Moria would be available later this month.
The ministry has ferried thousands of asylum-seekers to the mainland in past months, while others have been granted refugee status and allowed to travel off the islands.
But the island camps remain badly overcrowded.
Overall, there are nearly 24,000 people in five island camps built to handle fewer than 6,100.
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.
The infection rate accelerated in August, which accounted for more than half of Greece's 10,500 cases.
Authorities have blamed the spike in infections to the flouting of social distancing rules in restaurants, bars and public gatherings.
The civil protection agency has made masks compulsory in all indoor public areas.
The government has ruled out a general lockdown after gradually reopening the economy in May and starting to accept foreign arrivals in June to salvage part of the tourism season which is vital to the economy.