To make money online without having a passion for the topic you choose can be done.
People work at jobs they have no passion for but do so for the money. Before making a website on something you're not sure about then research, research and research some more.
Study up on the topics and get really educated about the things you're gonna write about especially health.
To keep motivated think about the money you can make in the long run. You'll be working for hours at a time without any pay. But if you keep up with the blogging and working on your website then you'll eventually see a big payoff.
But it's not enough to just write content. You should also promote the content on twitter,facebook, pinterest etc.
Making money online takes time and a lot of work so don't give up.