Tutu on Wednesday checked into a Cape Town hospital to receive treatment for an undisclosed "persistent infection" and to take tests to establish its underlying cause, according to his foundation.
"We are not going to issue updates on his condition because he is currently undergoing a five-day treatment. At the end of the course we will inform the media," a spokesperson said.
Known fondly as "the Arch" Tutu has survived an illness believed to be polio as a baby, battled tuberculosis as a teenager and in 1997 was diagnosed with prostrate cancer.
Nearly 10 years later, he said the cancer had returned after having gone into remission but was non-aggressive. In December 2011, he underwent minor elective surgery in Cape Town for an undisclosed complaint.
His recent public appearances have shown little hint of ill-health. Officially retired, the outspoken Tutu is still seen as South Africa's moral beacon.
He won the 1984 Nobel Peace Prize for his work, which he said has always been motivated by religion. – AFP