Outlining a timeline, Oceanwide Expeditions said a passenger had become unwell while aboard and died on 11 April.
His cause of death could not be determined, and his body was taken off the ship after it docked at St Helena on 24 April.
The passenger’s wife also disembarked on St Helena and the firm said it was told she had become unwell during the return journey and later died.
On 27 April, the firm said, another passenger – the British national – became seriously ill and was “medically evacuated” to South Africa.
The 69-year-old remains in a critical but stable condition in Johannesburg after it was confirmed a variant of hantavirus had been identified.
The firm added that on Saturday, a third passenger onboard MV Hondius died.
The cause of death has not been established, Oceanwide Expeditions said. It confirmed the passenger was German.
Dr Aaron Motsoaledi, South Africa’s minister of health, said of the British patient that he was critical and had been admitted to a private facililty.
“He’s being taken care of. As you know, hantavirus, like all viruses, don’t have any specific treatment, so they are giving symptomatic treatment and support as much as they could.”
He said health workers and anyone who had contact with the patient would now be traced and tested.
A UK government spokesperson said the Foreign Office was in touch with the man’s family and that consular teams were operating in the UK, South Africa, Spain and Portugal to support British nationals.
