Again, while no Orders exist, if the child has a minor medical ailment (such as flu) then you do not have to advise the other parent. However, if there is a medical emergency that requires hospitalisation (such as a broken arm) then it would be wise to let the other parent know as soon as practicable. While you technically do not have to advise them, any Court would look upon this behaviour as being extremely poor and it could backfire on you in the long term.