Earlier this year, Cassidy admitted that he was struggling with memory loss.
After being rushed to hospital in Florida earlier this week, the Partridge Family star has been suffering from organ failure.
"The Partridge Family", a sitcom about a mother and five children who formed a rock "n" roll band, gave Cassidy a national audience for his music.
Cassidy, aged 67, is "now conscious" and "surrounded by family", a representative said.
According to TMZ, the actor is looking at getting a liver transplant, without which "he could soon die" with the situation "looking grim".More news: European Union cuts United Kingdom growth forecast as eurozone motors ahead
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The singer toured the world singing his hit songs, such as "I Think I Love You", filling concert halls with screaming teenage girls.
Cassidy starred in the beloved "Partridge" series from 1970-1974, and the role made him a popular teen idol.
Other media sites have reported Cassidy is suffering from organ failure.
"I was in denial", Cassidy said, "but a part of me always knew this was coming".