Dr Hargrave's father Michael - later a GP himself in Royal Wootton Bassett, Wiltshire - was among 95 volunteers rounded up from six London medical schools in May 1945.
The group were told they would be going to recently liberated Holland to support starving civilians.
'But on the day of departure, they were told they were being sent to Belsen concentration camp,' says Dr Hargrave.
More than 65 years later, the daily journal his father kept, detailing his experiences at the camp, is being published.
'I've had the diary all my life,' says Dr Hargrave. 'My father kept it mainly for his mother, recording each day what he did, what conditions were like and the numbers who had died.'
Around 10,000 inmates who had died in the German camp had been buried by the British military, leaving the army medical authorities facing the challenge of saving survivors and preventing the spread of disease.
Each medical student volunteer, including a 21-year-old Michael Hargrave, was allocated a hut in the camp to manage. For a month, he improvised and did his best to meet the needs of the inmates of Belsen's hut 210.
The students' primary role was to 'clear the camp out, sort the living from the dead and try to stop people dying', says Dr Hargrave.
'It was a culture shock - he was only 21 and not highly qualified. The conditions were horrendous. He looked after mainly young women with typhus, diarrhoea, malnutrition.'
Dr Hargrave says he hopes publishing the journal will contribute to preventing a repeat of the horrors visited on prisoners held at Belsen. 'It is repeating itself around the world, but exposing the horror and disease in the book is to try to prevent this sort of thing happening again,' he says.
A copy of the journal was handed to the Imperial War Museum archive in 1968 by Dr Hargrave's father and has proved a rich resource for historians despite remaining unpublished in its own right until now.
In a foreword to the book, head of research at the Imperial War Museum Suzanne Bardgett writes: 'Hargrave concentrates mainly on the medical detail and the vital organisational challenges which ultimately saved many hundreds of lives.
'Diagrams provide a useful record of the layout of various huts and facilities including the human laundry, where, at 17 separate tables, four-strong teams of German nurses worked simultaneously on one patient, washing them and powdering them with the now-banned pesticide DDT.'
Dr Hargrave moved to have the book published on the advice of a librarian at Chelsea and Westminster Hospital's library after his retirement three years ago. All proceeds from the book will go to human rights charity Amnesty International and the End Polio Now campaign.
- 'Bergen-Belsen 1945: A Medical Student's Journal' is published by Imperial College Press.