The elderly are especially vulnerable to pneumonia, which can be fatal. Its symptoms include fever, chills, a cough, chest pain and shortness of breath. Many germs cause pneumonia.
Many South Africans are including Mandela in their prayers on the Easter weekend, and spokesman Maharaj said expressions of concern have poured in from around the world. On Thursday, President Barack Obama said he was worried about Mandela's health, but noted he was as strong physically as he has been in leadership and character.
Mandela became South Africa's first black president in 1994 after elections were held, bringing an end to the system of white racist rule known as apartheid. After his release from prison in 1990, Mandela was widely credited with averting even greater bloodshed by helping the country in the transition to democratic rule.
Zuma's office has said doctors were acting with extreme caution because of the Mandela's advanced age.
Mandela is a revered figure in his homeland, which has named buildings and many other places after him and uses his image on national bank notes. He is also seen around the world as a symbol of reconciliation.