A Japanese health worker on the cruise quarantined ship off Japan tested positive for the novel coronavirus, AFP reported.
The Diamond Princess got infected in late January. So far 174 people on board - plus the worker - have tested positive, out of around 500 to be tested.
The ship has produced more coronavirus cases than any country other than China.
A quarantine officer on the Diamond Princess cruise ship fell sick with the Wuhan coronavirus they were trying to help control, AFP reported.
It came as health officials in Japan confirmed that at least 174 of the ship's original 3,7000 passengers have tested positive for the disease.
The Diamond Princess has now contributed more coronavirus cases to the world total than any nation other than China.
After the first cases started to be found, a quarantine was imposed on the ship. It is due to expire on February 19.
At first only a handful of cases were identified, but each day has brought dozens of new ones.
Confirmed victims have been were transported off the ship to local hospitals for treatment.
Despite taking precautions, health workers are at risk of catching the virus themselves. In China many medics have fallen sick, and at least one has died from the virus.
AFP said four hospitalized people were in critical condition and are either in intensive care or on a ventilator.
Passengers still on board are confined to their cabins. Only those on the inner cabins have been allowed on to the main deck for fresh air, under strict instructions that they wear face masks and gloves and keep a distance from each other.
Many on board are concerned that staying on the ship is increasing their risk of catching the disease, which was on Tuesday given the formal name COVID-19.