USS Fort McHenry, a US Navy Whidbey Island-class dock landing ship, with over 700 sailors and Marines aboard, has spent the last two months at sea without a port-call. The reason? There has been an outbreak of parotitis, a virus with symptoms similar to the mumps.

Until CNN asked about the time the ship had been at sea, the Navy had not disclosed that the vessel was effectively being quarantined. The illness first broke out in December, with the most recent case being reported on March 9. So far, only about 25 of the more than 700 aboard have caught the virus.

CNN reports that the ship includes elements of the 22nd Marine Expeditionary Unit. Since the initial case was detected on December 22, 24 of the 25 patients have returned to duty.

After they became ill, the patients were quarantined and treated in the ship’s medical facility. Living and workspaces were disinfected. None of the personnel had to be medevaced off the ship and all are expected to make a full recovery.

However, a military medical team specializing in preventative medical care is expected to deploy in the coming days to make an assessment if further steps may be needed, according to the official.

A US military official tells CNN that when there are major disease outbreaks, a decision may be taken to halt port visits until 30 days after the last reported illness due to varying incubation periods.