The health of Singapore's severely ill founding father, Lee Kuan Yew, has "deteriorated further", the Prime Minister's Office said in a statement today. "Mr Lee Kuan Yew remains critically ill in the ICU and has deteriorated further," the statement read. A day earlier, Lee's condition was said to have worsened due to an infection. "He is on antibiotics. The doctors are closely monitoring his condition," said the statement, which was also posted on the Facebook page of his eldest son, Singapore Prime Minister Lee Hsien Loong, on Tuesday. "The infection is potentially treatable with antibiotics," said a Singapore doctor who declined to be named. The South China Morning Post has learned from well-placed sources that the elder Lee, 91, is "severely ill". He has been in the intensive care unit of Singapore General Hospital since February 5, on mechanical ventilation. Doctors have said that generally, any 91-year-old patient on a ventilator for about four weeks has a very guarded prognosis. The website of the US National Heart, Lung and Blood Institute says: "One of the most serious and common risks of being on a ventilator is pneumonia. The breathing tube that's put in your airway can allow bacteria to enter your lungs. "As a result, you may develop ventilator-associated pneumonia [VAP]. VAP is a major concern for people using ventilators because they're often already very sick. VAP is treated with antibiotics." Another Singapore doctor said pneumonia acquired while in hospital was a leading cause of death in hospitalised elderly patients, with an estimated mortality rate from 30 to 70 per cent. Prime Minister Lee's Facebook post last Friday said: "In the month since Mr Lee Kuan Yew was admitted to hospital, so many people and groups have sent him good wishes. We've arranged the cards in his office, to welcome him back when he's better." In February, the younger Lee underwent successful surgery to remove his cancerous prostate gland. He has since returned to work. According to medical opinion, there is a very high survival rate for early-stage prostate cancer. Lee Kuan Yew was prime minister of Singapore from 1959 to September 1963, while the city was under British colonial rule, and then until August 1965 when the Lion City was part of Malaysia. He was the first prime minister of independent Singapore from August 1965 to November 1990. He was then senior minister until August 2004 and minister mentor until May 2011.