His condition had deteriorated but he was now stable, a hospital official said.
Mr Sharon, who has been in a coma since suffering a major stroke in January, contracted a new infection that affected his heart, said David Weinberg, a spokesman for the Chaim Sheba Medical Centre near Tel Aviv.
"At this point, his condition is stable," Mr Weinberg said.
The hospital refused to comment further.
Experts have speculated that because of the severity of his stroke, Mr Sharon, Israel's prime minister from 2001 to 2006, is unlikely to recover.
Dr Barbara Paris, director of geriatrics at Maimonides Medical Centre in New York City, said that even with the best possible treatment, such an infection in Sharon's circumstances renders the situation critical.
"Mr Sharon's condition was precarious prior to development of this infection," said Dr Paris, who is not involved in Sharon's treatment.
The 78 year old has also undergone several extensive brain operations to stop cerebral haemorrhaging, in addition to more minor procedures.
He had a small stroke in December and was put on blood thinners before experiencing a severe brain haemorrhage on January 4.
After months in the Jerusalem hospital where he was initially treated, Mr Sharon was transferred to the long-term care facility at Sheba hospital in May.
He was rushed into intensive care in July for dialysis after his kidneys began failing, but was transferred back to Sheba after his condition improved.
Mr Sharon lapsed into a coma just months after he ended Israel's 38-year occupation of the Gaza Strip bolted his hard-line Likud Party to form the centrist Kadima faction.
After the stroke, Sharon's successor, Ehud Olmert, led Kadima to victory in a March 28 vote and became prime minister.