Some topics are naturally so big that you can always extend them with content and marketing. However, as a beginner, you might not be able to work on big projects. Choose a niche instead that is expandable. Let’s clarify the topic with an example. When you’re choosing fitness, women fitness, child health — these are big niches, and you can create plenty of content around them. Likewise, if you go for “Yoga outfits”, now that’s a microscopic niche.
Affiliate marketing is not a “get rich quick” scheme. But it is a simple model to earn an income once you know the steps. In order to be successful with affiliate marketing, you’ll need to execute three vital steps.

The medium you choose to base your business on will very much depend on the kind of person you are and your personal preferences. You need to play to your strengths here. If you’re a writer, write. If you prefer video as a means to communicate with your audience, then that’s the way to start. A podcast is a great option if you’re a confident and engaging speaker but don’t like being on video.
You can contact them and offer them your affiliate network. There is a sub-forum with the name “Online Clinics” where doctors participate and answer questions online.