Doctors are trying to work out if a flu outbreak, which has affected more than 90 inmates at a jail, played a part in a prisoner's death.
The inmate at Stafford Prison fell ill on Monday and was taken to hospital, where he died on Tuesday of pneumonia with complications.
Another prisoner is in a stable condition in hospital and is expected to make a full recovery.
Some staff have also been affected by the outbreak of flu.
Public health doctors from the Health Protection Agency (HPA) and the South Staffordshire Primary Care Trust are investigating the outbreak of flu.
It is likely there will be more cases at the 600-inmate prison in the coming week, the HPA said.
Inmates with flu symptoms at the jail are staying in their cells to avoid infecting others and are being checked on by prison healthcare workers and wardens.
South Staffordshire Primary Care Trust has provided additional nursing and medical support to the prison healthcare team, the HPA added.