It needs to be something that has been difficult/outside of your comfort zone but something you have learnt and grown from. It doesn’t need to come from work or your studies, it can come from life more generally.
Although some people do use health related matters to highlight challenges, you have to show the growth/learning from them. Sometimes it doesn’t work when it’s a personal matter like that as the circumstances just no longer exist (eg you get treated/recover), when you really need to show the development in yourself, not just the ability to deal with the challenge in the moment.