He doesn't have any particularly great influence on "the medical world" as far as I can tell. He certainly has influence over the Bill and Melinda Gates Foundation and how it spends its money (since he and his wife own it) and over an organisation called the Coalition for Epidemic Preparedness Innovations which they co-founded but I don't see that he's controlling how any other medical body goes about their business.How does a man with no specific medical or biological knowledge and training have such an influence on the medical world?
If you're talking about the conspiracy theories that his foundation tested unapproved vaccines on children in Africa and India or that there were excessive adverse results in either place, then both of those claims have been debunked by various fact-checking websites.If you look at his track record with vaccines in India and sub Saharan Africa then you can see why he wants that protection.
Here's a link to just one of them:
Did Bill Gates test unapproved vaccines? No.