Jailer Mahbubul Islam said the prison guards had been suffering from fever while performing duty at old jail at Nazim Uddin Road and their samples were sent to the Institute of Epidemiology, Disease Control and Research on April 28 where they were found infected with the coronavirus.
All of them were aged between 30 and 55 years, he said.
On April 21, a 60-year-old prison guard was first detected with the coronavirus while discharging duties at Dhaka Medical College Hospital, the jailer added.
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Later, he was taken to the Keraniganj 20-bed hospital for treatment.
Of the infected prison guards, six were taken to Mugda General Hospital while three to a hospital in Mirpur on Tuesday night.
Five jail guards were sent to home-quarantine, said Jailer Mahbubul.
However, the jail authorities have taken steps to keep the prison inmates free from coronavirus.