An employee of South Africa's blood service has been suspended after wrongly telling a student she was HIV positive to see how she would react.
The blood service has been embroiled in several controversies
She was on her way to class and had her mother's mobile phone, when the man phoned her up.
"I was utterly shocked and burst into tears," said the student, 19. "It was a very cruel joke. HIV/Aids is a serious matter that one cannot joke about."
The man then explained his "joke" and she hung up on him.
He later phoned back and apologised to the woman's mother.
"One tries to do something good, but I've taken my name off their list now," the student said.
Nicolette Duda from the South African National Blood Service (SANBS) said the matter was viewed very seriously and an inquiry had been launched.
She said she feared that it could put people off donating blood and stressed that anyone with "abnormalities" in their blood after screening was sent a letter informing them to visit a doctor.
The SANBS has been embroiled in a number of controversies, as it bids to screen blood for HIV in the country with the highest number of infections in the world - more than 4m people.
Earlier this year, gay activists were outraged after it said it would no longer accept blood donations from men who have sex with men.
Last year it was forced to change its screening procedures when it was revealed that they destroyed blood donated by black people, because it claimed they were at a high risk of HIV infection.
It emerged that President Thabo Mbeki's blood was destroyed because he was black and because his doctor had refused to complete the personal history questionnaire used to screen donors.
Mr Mbeki donated blood as part of a publicity campaign to persuade people to donate blood.