how do i make additional fields appear based on a users previos input in a checkbox

how do i make additional fields appear based on a users previos input in a checkbox

I have a checkbox field with various diagnosis. The can check more than one. What I want to do: If the user checks the NPH diagnosis checkbox, then a whole new set of fields should appear for him to fill out. If he doesn´t check that particular checkbox then the additional field wont appear.