The 42-year-old actress was admitted to Mumbai's Jaslok Hospital on Wednesday (November 28). While officials have refused to reveal details of her condition, it has been reported that the actress is suffering with ovarian cancer.
According to The Times of India, Jaslok hospital spokesperson Krishnakant Dasyam said: "Manisha is responding well to the treatment and she has been referred for a few tests today.
"Unless we review the results, the exact nature of her illness cannot be known. And commenting about it till then will be mere speculation."
The Bombay star has requested that the details of her condition remain confidential, while a hospital source has reported that Koirala is "weak but [doing] okay".
Members of the actress's family have been visiting her in hospital, while industry friends have spoken of their confidence in Koirala's recovery.
Jackie Shroff, who starred in several films with Koirala, said: "Woh manage kar legi biru. (She will manage.) She is not a quitter. She has battled so much more in her life that a mere illness won't be able to defeat her."
Director Ram Gopal Varma said: "I have not been able to communicate with her... I've been getting these text messages about her health that got me very worried."
Koirala made her film debut in 1991 in Subhash Ghai's Saudagar, and became established as a leading star with the release of Mani Ratnam's Bombay, Agnisakshi, Dil Se and 1942- A Love Story.
The actress, who was one of the first heroines to work with all the three Khans, recently made a comeback with Ram Gopal Varma's 3D horror Bhoot Returns following her divorce.
It was widely rumoured that Madhur Bhandarkar's movie Heroine was based on Koirala's fading career.