Two children have contracted HIV from blood transfusions at Red Cross Children's Hospital in the last year, despite attempts to ensure the safety of donated blood. One child, a two-year-old cancer patient, died soon after the treatment.
Doctors say the death was not related to HIV.
The second child, who had a transfusion at three months to treat anaemia, is receiving life-long anti-Aids therapy from the hospital.
"All the blood that we supply is tested for HIV," said Arthur Bird, the medical director of the Western Province Blood Transfusion Service.
"Unfortunately, if the donor is in the window period, where their infection does not show up in our tests, there is nothing we can do."
The service has provided blood for almost two-million transfusions in the past 15 years.
Only "two or three" cases of contaminated blood have been detected.
In this instance, the blood was donated in April last year by a regular donor not considered to be at high risk of contracting HIV.
As with everyone about to give blood, the donor signed a sworn statement that he had not participated in high-risk behaviour in the previous six months.
In this context, "high risk behaviour" is seen as any of the following: Men having sex with men; anyone having sex with a prostitute; starting a new sexual relationship; having sex with more than one person; or injecting yourself with drugs.
"The blood was negative, but when the donor came back two months later the blood he donated was positive," said Bird.
The transfusion service traced the blood donated earlier to Red Cross Children's Hospital, where records showed it had been shared between the two youngsters.
"The irony is that for children, to be extra careful, we only use regular donors," he said. "Unfortunately in this case it didn't help."
Both children had contracted HIV, the virus that causes Aids.
Malitha Ramiah, the hospital's chief medical superintendent, said the incident had been difficult for everyone involved.
"We know that this can happen, but it is very distressing when it happens to your patients," she said.
"By the time we found out, one of the patients had already died - although it (the death) was not related to HIV.
"When the other one started getting ill, I decided that the hospital was obliged to provide treatment to keep the patient healthy."
The transfusion service recently participated in an international pilot project to examine the use of another HIV test, known as Polymerase Chain Reaction (PCR), to screen blood.
Using the PCR reduces the so-called window period, during which HIV is not detectable until about two weeks after infection.
The present test takes four to six weeks to detect HIV in the blood.
Bird believes the PCR will become the new standard for transfusion services.
"These 'window-period donations' are experienced around the world, although they are a particular problem in South Africa because of the high rate of new infections," he said.
"We are negotiating with manufacturers to reduce the price of the PCR so that it can be used for all blood."
The blood transfusion service came under fire recently for discriminating against gay men - discrimination that Bird feels is justifiable.
"We believe we should be over-cautious when it comes to protecting patients," he said, "even if it offends some potential donors."
Another problem is donors who consider giving blood an alternative to having an Aids test.
"Donors have a responsibility not to give blood if it could be unsafe and to ask us if they have queries," Bird said.
Accidental infections also pose a problem for health services, as the decision on whether to provide costly treatment lies with individual hospitals, already struggling with budget constraints.
Some doctors believe it should be state policy.
"Technically, if the infection arose because of medical treatment, then the state has a responsibility to the patient," said Greg Hussey of the Child Health Unit.
"Where there is a risk associated with that treatment, even if it is a remote one, you can't expect the individual to take responsibility."