First of all, I have no answer for this. I can only tell you what I would do and have done in this situation.
There are a lot of books I want to write, a lot of topics I want to tackle, but most of the time I shy away because I feel I don't have enough knowledge about the topics and don't want to come off as a fraud.
However, there are many ways to help yourself feel more confident writing these challenging topics.
First, research. Do as much research as possible. I can not stress this enough. When you think you have researched enough, research some more. However, when you write, do not just spit out facts that you have memorized in your research. You want to make your writing feel authentic. Your readers will not care about your characters if you are just spewing facts.
Second, find people who have lived through those situations, diseases, times, whatever your topic. First hand account are priceless. If you want to write about a war and not just have a day to day account of that war, you will want to have a first hand account of it to get a feel for their emotions in the time and afterwards. Get first hand accounts, but do not steal their stories. Take inspiration from them, but do not rob them of their tales, their lives.
Tell your truth. Not having all the knowledge on a topic can be a hindrance, but it can also be helpful. Bringing a naivete to a topic can make the story more authentic, depending on the point of view and position and such of the character.
Know your characters and where they are at in their story. Then, do research and get first hand accounts accordingly. This may take years. Also, write only when you feel that you are confident that you are writing the story that you want. If you get ahead of yourself, you could end up getting frustrated and give up. However, on the flip side of that, if you start writing too early, your story could turn into something you never could have dreamed.
