This mantra is called Mrut-Sanjivani-Mahamrutyunjay-Gayatri-Shabari-Mantra !
This mantra was used for Laxmana when he was unconscious during war with Ravana in Ramayana ! Hanumanji got the Sanjivani Buti ( Vanaspati) and this was the mantra used by Vaidya- Sushen to cure Laxmana ! Since then when ever someone is crucial / ill physically - it is used for curing !
Please note this mantra has to be used alongwith all normal medication as suggested by Doctors !
BUT YES THIS MANTRA CERTAINLY USEFUL IN CURING THE ILLNESS SPIRITUALLY !